Provided by: perl-doc_5.38.2-3.2ubuntu0.1_all 
NAME
perlglossary - Perl Glossary
VERSION
version 5.20210520
DESCRIPTION
A glossary of terms (technical and otherwise) used in the Perl documentation, derived from the Glossary
of Programming Perl, Fourth Edition. Words or phrases in bold are defined elsewhere in this glossary.
Other useful sources include the Unicode Glossary <http://unicode.org/glossary/>, the Free On-Line
Dictionary of Computing <http://foldoc.org/>, the Jargon File <http://catb.org/~esr/jargon/>, and
Wikipedia <http://www.wikipedia.org/>.
A
accessor methods
A method used to indirectly inspect or update an object’s state (its instance variables).
actual arguments
The scalar values that you supply to a function or subroutine when you call it. For instance, when
you call power("puff"), the string "puff" is the actual argument. See also argument and formal
arguments.
address operator
Some languages work directly with the memory addresses of values, but this can be like playing with
fire. Perl provides a set of asbestos gloves for handling all memory management. The closest to an
address operator in Perl is the backslash operator, but it gives you a hard reference, which is much
safer than a memory address.
algorithm
A well-defined sequence of steps, explained clearly enough that even a computer could do them.
alias
A nickname for something, which behaves in all ways as though you’d used the original name instead of
the nickname. Temporary aliases are implicitly created in the loop variable for "foreach" loops, in
the $_ variable for "map" or "grep" operators, in $a and $b during "sort"’s comparison function, and
in each element of @_ for the actual arguments of a subroutine call. Permanent aliases are explicitly
created in packages by importing symbols or by assignment to typeglobs. Lexically scoped aliases for
package variables are explicitly created by the "our" declaration.
alphabetic
The sort of characters we put into words. In Unicode, this is all letters including all ideographs
and certain diacritics, letter numbers like Roman numerals, and various combining marks.
alternatives
A list of possible choices from which you may select only one, as in, “Would you like door A, B, or
C?” Alternatives in regular expressions are separated with a single vertical bar: "|". Alternatives
in normal Perl expressions are separated with a double vertical bar: "||". Logical alternatives in
Boolean expressions are separated with either "||" or "or".
anonymous
Used to describe a referent that is not directly accessible through a named variable. Such a referent
must be indirectly accessible through at least one hard reference. When the last hard reference goes
away, the anonymous referent is destroyed without pity.
application
A bigger, fancier sort of program with a fancier name so people don’t realize they are using a
program.
architecture
The kind of computer you’re working on, where one “kind of computer” means all those computers
sharing a compatible machine language. Since Perl programs are (typically) simple text files, not
executable images, a Perl program is much less sensitive to the architecture it’s running on than
programs in other languages, such as C, that are compiled into machine code. See also platform and
operating system.
argument
A piece of data supplied to a program, subroutine, function, or method to tell it what it’s supposed
to do. Also called a “parameter”.
ARGV
The name of the array containing the argument vector from the command line. If you use the empty "<>"
operator, "ARGV" is the name of both the filehandle used to traverse the arguments and the scalar
containing the name of the current input file.
arithmetical operator
A symbol such as "+" or "/" that tells Perl to do the arithmetic you were supposed to learn in grade
school.
array
An ordered sequence of values, stored such that you can easily access any of the values using an
integer subscript that specifies the value’s offset in the sequence.
array context
An archaic expression for what is more correctly referred to as list context.
Artistic License
The open source license that Larry Wall created for Perl, maximizing Perl’s usefulness, availability,
and modifiability. The current version is 2.
(<http://www.opensource.org/licenses/artistic-license.php>).
ASCII
The American Standard Code for Information Interchange (a 7-bit character set adequate only for
poorly representing English text). Often used loosely to describe the lowest 128 values of the
various ISO-8859-X character sets, a bunch of mutually incompatible 8-bit codes best described as
half ASCII. See also Unicode.
assertion
A component of a regular expression that must be true for the pattern to match but does not
necessarily match any characters itself. Often used specifically to mean a zero-width assertion.
assignment
An operator whose assigned mission in life is to change the value of a variable.
assignment operator
Either a regular assignment or a compound operator composed of an ordinary assignment and some other
operator, that changes the value of a variable in place; that is, relative to its old value. For
example, "$a += 2" adds 2 to $a.
associative array
See hash. Please. The term associative array is the old Perl 4 term for a hash. Some languages call
it a dictionary.
associativity
Determines whether you do the left operator first or the right operator first when you have “A
operator B operator C”, and the two operators are of the same precedence. Operators like "+" are left
associative, while operators like "**" are right associative. See Camel chapter 3, “Unary and Binary
Operators” for a list of operators and their associativity.
asynchronous
Said of events or activities whose relative temporal ordering is indeterminate because too many
things are going on at once. Hence, an asynchronous event is one you didn’t know when to expect.
atom
A regular expression component potentially matching a substring containing one or more characters and
treated as an indivisible syntactic unit by any following quantifier. (Contrast with an assertion
that matches something of zero width and may not be quantified.)
atomic operation
When Democritus gave the word “atom” to the indivisible bits of matter, he meant literally something
that could not be cut: ἀ- (not) + -τομος (cuttable). An atomic operation is an action that can’t be
interrupted, not one forbidden in a nuclear-free zone.
attribute
A new feature that allows the declaration of variables and subroutines with modifiers, as in "sub foo
: locked method". Also another name for an instance variable of an object.
autogeneration
A feature of operator overloading of objects, whereby the behavior of certain operators can be
reasonably deduced using more fundamental operators. This assumes that the overloaded operators will
often have the same relationships as the regular operators. See Camel chapter 13, “Overloading”.
autoincrement
To add one to something automatically, hence the name of the "++" operator. To instead subtract one
from something automatically is known as an “autodecrement”.
autoload
To load on demand. (Also called “lazy” loading.) Specifically, to call an "AUTOLOAD" subroutine on
behalf of an undefined subroutine.
autosplit
To split a string automatically, as the –a switch does when running under –p or –n in order to
emulate awk. (See also the "AutoSplit" module, which has nothing to do with the "–a" switch but a lot
to do with autoloading.)
autovivification
A Graeco-Roman word meaning “to bring oneself to life”. In Perl, storage locations (lvalues)
spontaneously generate themselves as needed, including the creation of any hard reference values to
point to the next level of storage. The assignment "$a[5][5][5][5][5] = "quintet"" potentially
creates five scalar storage locations, plus four references (in the first four scalar locations)
pointing to four new anonymous arrays (to hold the last four scalar locations). But the point of
autovivification is that you don’t have to worry about it.
AV Short for “array value”, which refers to one of Perl’s internal data types that holds an array. The
"AV" type is a subclass of SV.
awk Descriptive editing term—short for “awkward”. Also coincidentally refers to a venerable text-
processing language from which Perl derived some of its high-level ideas.
B
backreference
A substring captured by a subpattern within unadorned parentheses in a regex. Backslashed decimal
numbers ("\1", "\2", etc.) later in the same pattern refer back to the corresponding subpattern in
the current match. Outside the pattern, the numbered variables ($1, $2, etc.) continue to refer to
these same values, as long as the pattern was the last successful match of the current dynamic scope.
backtracking
The practice of saying, “If I had to do it all over, I’d do it differently,” and then actually going
back and doing it all over differently. Mathematically speaking, it’s returning from an unsuccessful
recursion on a tree of possibilities. Perl backtracks when it attempts to match patterns with a
regular expression, and its earlier attempts don’t pan out. See the section “The Little Engine That
/Couldn(n’t)” in Camel chapter 5, “Pattern Matching”.
backward compatibility
Means you can still run your old program because we didn’t break any of the features or bugs it was
relying on.
bareword
A word sufficiently ambiguous to be deemed illegal under "use strict 'subs'". In the absence of that
stricture, a bareword is treated as if quotes were around it.
base class
A generic object type; that is, a class from which other, more specific classes are derived
genetically by inheritance. Also called a “superclass” by people who respect their ancestors.
big-endian
From Swift: someone who eats eggs big end first. Also used of computers that store the most
significant byte of a word at a lower byte address than the least significant byte. Often considered
superior to little-endian machines. See also little-endian.
binary
Having to do with numbers represented in base 2. That means there’s basically two numbers: 0 and 1.
Also used to describe a file of “nontext”, presumably because such a file makes full use of all the
binary bits in its bytes. With the advent of Unicode, this distinction, already suspect, loses even
more of its meaning.
binary operator
An operator that takes two operands.
bind
To assign a specific network address to a socket.
bit An integer in the range from 0 to 1, inclusive. The smallest possible unit of information storage. An
eighth of a byte or of a dollar. (The term “Pieces of Eight” comes from being able to split the old
Spanish dollar into 8 bits, each of which still counted for money. That’s why a 25- cent piece today
is still “two bits”.)
bit shift
The movement of bits left or right in a computer word, which has the effect of multiplying or
dividing by a power of 2.
bit string
A sequence of bits that is actually being thought of as a sequence of bits, for once.
bless
In corporate life, to grant official approval to a thing, as in, “The VP of Engineering has blessed
our WebCruncher project.” Similarly, in Perl, to grant official approval to a referent so that it can
function as an object, such as a WebCruncher object. See the "bless" function in Camel chapter 27,
“Functions”.
block
What a process does when it has to wait for something: “My process blocked waiting for the disk.” As
an unrelated noun, it refers to a large chunk of data, of a size that the operating system likes to
deal with (normally a power of 2 such as 512 or 8192). Typically refers to a chunk of data that’s
coming from or going to a disk file.
BLOCK
A syntactic construct consisting of a sequence of Perl statements that is delimited by braces. The
"if" and "while" statements are defined in terms of "BLOCK"s, for instance. Sometimes we also say
“block” to mean a lexical scope; that is, a sequence of statements that acts like a "BLOCK", such as
within an "eval" or a file, even though the statements aren’t delimited by braces.
block buffering
A method of making input and output efficient by passing one block at a time. By default, Perl does
block buffering to disk files. See buffer and command buffering.
Boolean
A value that is either true or false.
Boolean context
A special kind of scalar context used in conditionals to decide whether the scalar value returned by
an expression is true or false. Does not evaluate as either a string or a number. See context.
breakpoint
A spot in your program where you’ve told the debugger to stop execution so you can poke around and
see whether anything is wrong yet.
broadcast
To send a datagram to multiple destinations simultaneously.
BSD A psychoactive drug, popular in the ’80s, probably developed at UC Berkeley or thereabouts. Similar
in many ways to the prescription-only medication called “System V”, but infinitely more useful. (Or,
at least, more fun.) The full chemical name is “Berkeley Standard Distribution”.
bucket
A location in a hash table containing (potentially) multiple entries whose keys “hash” to the same
hash value according to its hash function. (As internal policy, you don’t have to worry about it
unless you’re into internals, or policy.)
buffer
A temporary holding location for data. Data that are Block buffering means that the data is passed on
to its destination whenever the buffer is full. Line buffering means that it’s passed on whenever a
complete line is received. Command buffering means that it’s passed every time you do a "print"
command (or equivalent). If your output is unbuffered, the system processes it one byte at a time
without the use of a holding area. This can be rather inefficient.
built-in
A function that is predefined in the language. Even when hidden by overriding, you can always get at
a built- in function by qualifying its name with the "CORE::" pseudopackage.
bundle
A group of related modules on CPAN. (Also sometimes refers to a group of command-line switches
grouped into one switch cluster.)
byte
A piece of data worth eight bits in most places.
bytecode
A pidgin-like lingo spoken among ’droids when they don’t wish to reveal their orientation (see
endian). Named after some similar languages spoken (for similar reasons) between compilers and
interpreters in the late 20ᵗʰ century. These languages are characterized by representing everything
as a nonarchitecture-dependent sequence of bytes.
C
C A language beloved by many for its inside-out type definitions, inscrutable precedence rules, and
heavy overloading of the function-call mechanism. (Well, actually, people first switched to C because
they found lowercase identifiers easier to read than upper.) Perl is written in C, so it’s not
surprising that Perl borrowed a few ideas from it.
cache
A data repository. Instead of computing expensive answers several times, compute it once and save the
result.
callback
A handler that you register with some other part of your program in the hope that the other part of
your program will trigger your handler when some event of interest transpires.
call by reference
An argument-passing mechanism in which the formal arguments refer directly to the actual arguments,
and the subroutine can change the actual arguments by changing the formal arguments. That is, the
formal argument is an alias for the actual argument. See also call by value.
call by value
An argument-passing mechanism in which the formal arguments refer to a copy of the actual arguments,
and the subroutine cannot change the actual arguments by changing the formal arguments. See also call
by reference.
canonical
Reduced to a standard form to facilitate comparison.
capture variables
The variables—such as $1 and $2, and "%+" and "%– "—that hold the text remembered in a pattern match.
See Camel chapter 5, “Pattern Matching”.
capturing
The use of parentheses around a subpattern in a regular expression to store the matched substring as
a backreference. (Captured strings are also returned as a list in list context.) See Camel chapter 5,
“Pattern Matching”.
cargo cult
Copying and pasting code without understanding it, while superstitiously believing in its value. This
term originated from preindustrial cultures dealing with the detritus of explorers and colonizers of
technologically advanced cultures. See The Gods Must Be Crazy.
case
A property of certain characters. Originally, typesetter stored capital letters in the upper of two
cases and small letters in the lower one. Unicode recognizes three cases: lowercase (character
property "\p{lower}"), titlecase ("\p{title}"), and uppercase ("\p{upper}"). A fourth casemapping
called foldcase is not itself a distinct case, but it is used internally to implement casefolding.
Not all letters have case, and some nonletters have case.
casefolding
Comparing or matching a string case-insensitively. In Perl, it is implemented with the "/i" pattern
modifier, the "fc" function, and the "\F" double-quote translation escape.
casemapping
The process of converting a string to one of the four Unicode casemaps; in Perl, it is implemented
with the "fc", "lc", "ucfirst", and "uc" functions.
character
The smallest individual element of a string. Computers store characters as integers, but Perl lets
you operate on them as text. The integer used to represent a particular character is called that
character’s codepoint.
character class
A square-bracketed list of characters used in a regular expression to indicate that any character of
the set may occur at a given point. Loosely, any predefined set of characters so used.
character property
A predefined character class matchable by the "\p" or "\P" metasymbol. Unicode defines hundreds of
standard properties for every possible codepoint, and Perl defines a few of its own, too.
circumfix operator
An operator that surrounds its operand, like the angle operator, or parentheses, or a hug.
class
A user-defined type, implemented in Perl via a package that provides (either directly or by
inheritance) methods (that is, subroutines) to handle instances of the class (its objects). See also
inheritance.
class method
A method whose invocant is a package name, not an object reference. A method associated with the
class as a whole. Also see instance method.
client
In networking, a process that initiates contact with a server process in order to exchange data and
perhaps receive a service.
closure
An anonymous subroutine that, when a reference to it is generated at runtime, keeps track of the
identities of externally visible lexical variables, even after those lexical variables have
supposedly gone out of scope. They’re called “closures” because this sort of behavior gives
mathematicians a sense of closure.
cluster
A parenthesized subpattern used to group parts of a regular expression into a single atom.
CODE
The word returned by the "ref" function when you apply it to a reference to a subroutine. See also
CV.
code generator
A system that writes code for you in a low-level language, such as code to implement the backend of a
compiler. See program generator.
codepoint
The integer a computer uses to represent a given character. ASCII codepoints are in the range 0 to
127; Unicode codepoints are in the range 0 to 0x1F_FFFF; and Perl codepoints are in the range 0 to
2³²−1 or 0 to 2⁶⁴−1, depending on your native integer size. In Perl Culture, sometimes called
ordinals.
code subpattern
A regular expression subpattern whose real purpose is to execute some Perl code—for example, the
"(?{...})" and "(??{...})" subpatterns.
collating sequence
The order into which characters sort. This is used by string comparison routines to decide, for
example, where in this glossary to put “collating sequence”.
co-maintainer
A person with permissions to index a namespace in PAUSE. Anyone can upload any namespace, but only
primary and co-maintainers get their contributions indexed.
combining character
Any character with the General Category of Combining Mark ("\p{GC=M}"), which may be spacing or
nonspacing. Some are even invisible. A sequence of combining characters following a grapheme base
character together make up a single user-visible character called a grapheme. Most but not all
diacritics are combining characters, and vice versa.
command
In shell programming, the syntactic combination of a program name and its arguments. More loosely,
anything you type to a shell (a command interpreter) that starts it doing something. Even more
loosely, a Perl statement, which might start with a label and typically ends with a semicolon.
command buffering
A mechanism in Perl that lets you store up the output of each Perl command and then flush it out as a
single request to the operating system. It’s enabled by setting the $| ($AUTOFLUSH) variable to a
true value. It’s used when you don’t want data sitting around, not going where it’s supposed to,
which may happen because the default on a file or pipe is to use block buffering.
command-line arguments
The values you supply along with a program name when you tell a shell to execute a command. These
values are passed to a Perl program through @ARGV.
command name
The name of the program currently executing, as typed on the command line. In C, the command name is
passed to the program as the first command-line argument. In Perl, it comes in separately as $0.
comment
A remark that doesn’t affect the meaning of the program. In Perl, a comment is introduced by a "#"
character and continues to the end of the line.
compilation unit
The file (or string, in the case of "eval") that is currently being compiled.
compile
The process of turning source code into a machine-usable form. See compile phase.
compile phase
Any time before Perl starts running your main program. See also run phase. Compile phase is mostly
spent in compile time, but may also be spent in runtime when "BEGIN" blocks, "use" or "no"
declarations, or constant subexpressions are being evaluated. The startup and import code of any
"use" declaration is also run during compile phase.
compiler
Strictly speaking, a program that munches up another program and spits out yet another file
containing the program in a “more executable” form, typically containing native machine instructions.
The perl program is not a compiler by this definition, but it does contain a kind of compiler that
takes a program and turns it into a more executable form (syntax trees) within the perl process
itself, which the interpreter then interprets. There are, however, extension modules to get Perl to
act more like a “real” compiler. See Camel chapter 16, “Compiling”.
compile time
The time when Perl is trying to make sense of your code, as opposed to when it thinks it knows what
your code means and is merely trying to do what it thinks your code says to do, which is runtime.
composer
A “constructor” for a referent that isn’t really an object, like an anonymous array or a hash (or a
sonata, for that matter). For example, a pair of braces acts as a composer for a hash, and a pair of
brackets acts as a composer for an array. See the section “Creating References” in Camel chapter 8,
“References”.
concatenation
The process of gluing one cat’s nose to another cat’s tail. Also a similar operation on two strings.
conditional
Something “iffy”. See Boolean context.
connection
In telephony, the temporary electrical circuit between the caller’s and the callee’s phone. In
networking, the same kind of temporary circuit between a client and a server.
construct
As a noun, a piece of syntax made up of smaller pieces. As a transitive verb, to create an object
using a constructor.
constructor
Any class method, instance, or subroutine that composes, initializes, blesses, and returns an object.
Sometimes we use the term loosely to mean a composer.
context
The surroundings or environment. The context given by the surrounding code determines what kind of
data a particular expression is expected to return. The three primary contexts are list context,
scalar, and void context. Scalar context is sometimes subdivided into Boolean context, numeric
context, string context, and void context. There’s also a “don’t care” context (which is dealt with
in Camel chapter 2, “Bits and Pieces”, if you care).
continuation
The treatment of more than one physical line as a single logical line. Makefile lines are continued
by putting a backslash before the newline. Mail headers, as defined by RFC 822, are continued by
putting a space or tab after the newline. In general, lines in Perl do not need any form of
continuation mark, because whitespace (including newlines) is gleefully ignored. Usually.
core dump
The corpse of a process, in the form of a file left in the working directory of the process, usually
as a result of certain kinds of fatal errors.
CPAN
The Comprehensive Perl Archive Network. (See the Camel Preface and Camel chapter 19, “CPAN” for
details.)
C preprocessor
The typical C compiler’s first pass, which processes lines beginning with "#" for conditional
compilation and macro definition, and does various manipulations of the program text based on the
current definitions. Also known as cpp(1).
cracker
Someone who breaks security on computer systems. A cracker may be a true hacker or only a script
kiddie.
currently selected output channel
The last filehandle that was designated with select(FILEHANDLE); "STDOUT", if no filehandle has been
selected.
current package
The package in which the current statement is compiled. Scan backward in the text of your program
through the current lexical scope or any enclosing lexical scopes until you find a package
declaration. That’s your current package name.
current working directory
See working directory.
CV In academia, a curriculum vitæ, a fancy kind of résumé. In Perl, an internal “code value” typedef
holding a subroutine. The "CV" type is a subclass of SV.
D
dangling statement
A bare, single statement, without any braces, hanging off an "if" or "while" conditional. C allows
them. Perl doesn’t.
datagram
A packet of data, such as a UDP message, that (from the viewpoint of the programs involved) can be
sent independently over the network. (In fact, all packets are sent independently at the IP level,
but stream protocols such as TCP hide this from your program.)
data structure
How your various pieces of data relate to each other and what shape they make when you put them all
together, as in a rectangular table or a triangular tree.
data type
A set of possible values, together with all the operations that know how to deal with those values.
For example, a numeric data type has a certain set of numbers that you can work with, as well as
various mathematical operations that you can do on the numbers, but would make little sense on, say,
a string such as "Kilroy". Strings have their own operations, such as concatenation. Compound types
made of a number of smaller pieces generally have operations to compose and decompose them, and
perhaps to rearrange them. Objects that model things in the real world often have operations that
correspond to real activities. For instance, if you model an elevator, your elevator object might
have an "open_door" method.
DBM Stands for “Database Management” routines, a set of routines that emulate an associative array using
disk files. The routines use a dynamic hashing scheme to locate any entry with only two disk
accesses. DBM files allow a Perl program to keep a persistent hash across multiple invocations. You
can "tie" your hash variables to various DBM implementations.
declaration
An assertion that states something exists and perhaps describes what it’s like, without giving any
commitment as to how or where you’ll use it. A declaration is like the part of your recipe that says,
“two cups flour, one large egg, four or five tadpoles…” See statement for its opposite. Note that
some declarations also function as statements. Subroutine declarations also act as definitions if a
body is supplied.
declarator
Something that tells your program what sort of variable you’d like. Perl doesn’t require you to
declare variables, but you can use "my", "our", or "state" to denote that you want something other
than the default.
decrement
To subtract a value from a variable, as in “decrement $x” (meaning to remove 1 from its value) or
“decrement $x by 3”.
default
A value chosen for you if you don’t supply a value of your own.
defined
Having a meaning. Perl thinks that some of the things people try to do are devoid of meaning; in
particular, making use of variables that have never been given a value and performing certain
operations on data that isn’t there. For example, if you try to read data past the end of a file,
Perl will hand you back an undefined value. See also false and the "defined" entry in Camel chapter
27, “Functions”.
delimiter
A character or string that sets bounds to an arbitrarily sized textual object, not to be confused
with a separator or terminator. “To delimit” really just means “to surround” or “to enclose” (like
these parentheses are doing).
dereference
A fancy computer science term meaning “to follow a reference to what it points to”. The “de” part of
it refers to the fact that you’re taking away one level of indirection.
derived class
A class that defines some of its methods in terms of a more generic class, called a base class. Note
that classes aren’t classified exclusively into base classes or derived classes: a class can function
as both a derived class and a base class simultaneously, which is kind of classy.
descriptor
See file descriptor.
destroy
To deallocate the memory of a referent (first triggering its "DESTROY" method, if it has one).
destructor
A special method that is called when an object is thinking about destroying itself. A Perl program’s
"DESTROY" method doesn’t do the actual destruction; Perl just triggers the method in case the class
wants to do any associated cleanup.
device
A whiz-bang hardware gizmo (like a disk or tape drive or a modem or a joystick or a mouse) attached
to your computer, which the operating system tries to make look like a file (or a bunch of files).
Under Unix, these fake files tend to live in the /dev directory.
directive
A pod directive. See Camel chapter 23, “Plain Old Documentation”.
directory
A special file that contains other files. Some operating systems call these “folders”, “drawers”,
“catalogues”, or “catalogs”.
directory handle
A name that represents a particular instance of opening a directory to read it, until you close it.
See the "opendir" function.
discipline
Some people need this and some people avoid it. For Perl, it’s an old way to say I/O layer.
dispatch
To send something to its correct destination. Often used metaphorically to indicate a transfer of
programmatic control to a destination selected algorithmically, often by lookup in a table of
function references or, in the case of object methods, by traversing the inheritance tree looking for
the most specific definition for the method.
distribution
A standard, bundled release of a system of software. The default usage implies source code is
included. If that is not the case, it will be called a “binary-only” distribution.
dual-lived
Some modules live both in the Standard Library and on CPAN. These modules might be developed on two
tracks as people modify either version. The trend currently is to untangle these situations.
dweomer
An enchantment, illusion, phantasm, or jugglery. Said when Perl’s magical dwimmer effects don’t do
what you expect, but rather seem to be the product of arcane dweomercraft, sorcery, or wonder
working. [From Middle English.]
dwimmer
DWIM is an acronym for “Do What I Mean”, the principle that something should just do what you want it
to do without an undue amount of fuss. A bit of code that does “dwimming” is a “dwimmer”. Dwimming
can require a great deal of behind-the-scenes magic, which (if it doesn’t stay properly behind the
scenes) is called a dweomer instead.
dynamic scoping
Dynamic scoping works over a dynamic scope, making variables visible throughout the rest of the block
in which they are first used and in any subroutines that are called by the rest of the block.
Dynamically scoped variables can have their values temporarily changed (and implicitly restored
later) by a "local" operator. (Compare lexical scoping.) Used more loosely to mean how a subroutine
that is in the middle of calling another subroutine “contains” that subroutine at runtime.
E
eclectic
Derived from many sources. Some would say too many.
element
A basic building block. When you’re talking about an array, it’s one of the items that make up the
array.
embedding
When something is contained in something else, particularly when that might be considered surprising:
“I’ve embedded a complete Perl interpreter in my editor!”
empty subclass test
The notion that an empty derived class should behave exactly like its base class.
encapsulation
The veil of abstraction separating the interface from the implementation (whether enforced or not),
which mandates that all access to an object’s state be through methods alone.
endian
See little-endian and big-endian.
en passant
When you change a value as it is being copied. [From French “in passing”, as in the exotic pawn-
capturing maneuver in chess.]
environment
The collective set of environment variables your process inherits from its parent. Accessed via %ENV.
environment variable
A mechanism by which some high-level agent such as a user can pass its preferences down to its future
offspring (child processes, grandchild processes, great-grandchild processes, and so on). Each
environment variable is a key/value pair, like one entry in a hash.
EOF End of File. Sometimes used metaphorically as the terminating string of a here document.
errno
The error number returned by a syscall when it fails. Perl refers to the error by the name $! (or
$OS_ERROR if you use the English module).
error
See exception or fatal error.
escape sequence
See metasymbol.
exception
A fancy term for an error. See fatal error.
exception handling
The way a program responds to an error. The exception-handling mechanism in Perl is the "eval"
operator.
exec
To throw away the current process’s program and replace it with another, without exiting the process
or relinquishing any resources held (apart from the old memory image).
executable file
A file that is specially marked to tell the operating system that it’s okay to run this file as a
program. Usually shortened to “executable”.
execute
To run a program or subroutine. (Has nothing to do with the "kill" built-in, unless you’re trying to
run a signal handler.)
execute bit
The special mark that tells the operating system it can run this program. There are actually three
execute bits under Unix, and which bit gets used depends on whether you own the file singularly,
collectively, or not at all.
exit status
See status.
exploit
Used as a noun in this case, this refers to a known way to compromise a program to get it to do
something the author didn’t intend. Your task is to write unexploitable programs.
export
To make symbols from a module available for import by other modules.
expression
Anything you can legally say in a spot where a value is required. Typically composed of literals,
variables, operators, functions, and subroutine calls, not necessarily in that order.
extension
A Perl module that also pulls in compiled C or C++ code. More generally, any experimental option that
can be compiled into Perl, such as multithreading.
F
false
In Perl, any value that would look like "" or "0" if evaluated in a string context. Since undefined
values evaluate to "", all undefined values are false, but not all false values are undefined.
FAQ Frequently Asked Question (although not necessarily frequently answered, especially if the answer
appears in the Perl FAQ shipped standard with Perl).
fatal error
An uncaught exception, which causes termination of the process after printing a message on your
standard error stream. Errors that happen inside an "eval" are not fatal. Instead, the "eval"
terminates after placing the exception message in the $@ ($EVAL_ERROR) variable. You can try to
provoke a fatal error with the "die" operator (known as throwing or raising an exception), but this
may be caught by a dynamically enclosing "eval". If not caught, the "die" becomes a fatal error.
feeping creaturism
A spoonerism of “creeping featurism”, noting the biological urge to add just one more feature to a
program.
field
A single piece of numeric or string data that is part of a longer string, record, or line. Variable-
width fields are usually split up by separators (so use "split" to extract the fields), while fixed-
width fields are usually at fixed positions (so use "unpack"). Instance variables are also known as
“fields”.
FIFO
First In, First Out. See also LIFO. Also a nickname for a named pipe.
file
A named collection of data, usually stored on disk in a directory in a filesystem. Roughly like a
document, if you’re into office metaphors. In modern filesystems, you can actually give a file more
than one name. Some files have special properties, like directories and devices.
file descriptor
The little number the operating system uses to keep track of which opened file you’re talking about.
Perl hides the file descriptor inside a standard I/O stream and then attaches the stream to a
filehandle.
fileglob
A “wildcard” match on filenames. See the "glob" function.
filehandle
An identifier (not necessarily related to the real name of a file) that represents a particular
instance of opening a file, until you close it. If you’re going to open and close several different
files in succession, it’s fine to open each of them with the same filehandle, so you don’t have to
write out separate code to process each file.
filename
One name for a file. This name is listed in a directory. You can use it in an "open" to tell the
operating system exactly which file you want to open, and associate the file with a filehandle, which
will carry the subsequent identity of that file in your program, until you close it.
filesystem
A set of directories and files residing on a partition of the disk. Sometimes known as a “partition”.
You can change the file’s name or even move a file around from directory to directory within a
filesystem without actually moving the file itself, at least under Unix.
file test operator
A built-in unary operator that you use to determine whether something is true about a file, such as
–o $filename to test whether you’re the owner of the file.
filter
A program designed to take a stream of input and transform it into a stream of output.
first-come
The first PAUSE author to upload a namespace automatically becomes the primary maintainer for that
namespace. The “first come” permissions distinguish a primary maintainer who was assigned that role
from one who received it automatically.
flag
We tend to avoid this term because it means so many things. It may mean a command-line switch that
takes no argument itself (such as Perl’s "–n" and "–p" flags) or, less frequently, a single-bit
indicator (such as the "O_CREAT" and "O_EXCL" flags used in "sysopen"). Sometimes informally used to
refer to certain regex modifiers.
floating point
A method of storing numbers in “scientific notation”, such that the precision of the number is
independent of its magnitude (the decimal point “floats”). Perl does its numeric work with floating-
point numbers (sometimes called “floats”) when it can’t get away with using integers. Floating-point
numbers are mere approximations of real numbers.
flush
The act of emptying a buffer, often before it’s full.
FMTEYEWTK
Far More Than Everything You Ever Wanted To Know. An exhaustive treatise on one narrow topic,
something of a super-FAQ. See Tom for far more.
foldcase
The casemap used in Unicode when comparing or matching without regard to case. Comparing lower-,
title-, or uppercase are all unreliable due to Unicode’s complex, one-to-many case mappings. Foldcase
is a lowercase variant (using a partially decomposed normalization form for certain codepoints)
created specifically to resolve this.
fork
To create a child process identical to the parent process at its moment of conception, at least until
it gets ideas of its own. A thread with protected memory.
formal arguments
The generic names by which a subroutine knows its arguments. In many languages, formal arguments are
always given individual names; in Perl, the formal arguments are just the elements of an array. The
formal arguments to a Perl program are $ARGV[0], $ARGV[1], and so on. Similarly, the formal arguments
to a Perl subroutine are $_[0], $_[1], and so on. You may give the arguments individual names by
assigning the values to a "my" list. See also actual arguments.
format
A specification of how many spaces and digits and things to put somewhere so that whatever you’re
printing comes out nice and pretty.
freely available
Means you don’t have to pay money to get it, but the copyright on it may still belong to someone else
(like Larry).
freely redistributable
Means you’re not in legal trouble if you give a bootleg copy of it to your friends and we find out
about it. In fact, we’d rather you gave a copy to all your friends.
freeware
Historically, any software that you give away, particularly if you make the source code available as
well. Now often called open source software. Recently there has been a trend to use the term in
contradistinction to open source software, to refer only to free software released under the Free
Software Foundation’s GPL (General Public License), but this is difficult to justify etymologically.
function
Mathematically, a mapping of each of a set of input values to a particular output value. In
computers, refers to a subroutine or operator that returns a value. It may or may not have input
values (called arguments).
funny character
Someone like Larry, or one of his peculiar friends. Also refers to the strange prefixes that Perl
requires as noun markers on its variables.
G
garbage collection
A misnamed feature—it should be called, “expecting your mother to pick up after you”. Strictly
speaking, Perl doesn’t do this, but it relies on a reference-counting mechanism to keep things tidy.
However, we rarely speak strictly and will often refer to the reference-counting scheme as a form of
garbage collection. (If it’s any comfort, when your interpreter exits, a “real” garbage collector
runs to make sure everything is cleaned up if you’ve been messy with circular references and such.)
GID Group ID—in Unix, the numeric group ID that the operating system uses to identify you and members of
your group.
glob
Strictly, the shell’s "*" character, which will match a “glob” of characters when you’re trying to
generate a list of filenames. Loosely, the act of using globs and similar symbols to do pattern
matching. See also fileglob and typeglob.
global
Something you can see from anywhere, usually used of variables and subroutines that are visible
everywhere in your program. In Perl, only certain special variables are truly global—most variables
(and all subroutines) exist only in the current package. Global variables can be declared with
"our". See “Global Declarations” in Camel chapter 4, “Statements and Declarations”.
global destruction
The garbage collection of globals (and the running of any associated object destructors) that takes
place when a Perl interpreter is being shut down. Global destruction should not be confused with the
Apocalypse, except perhaps when it should.
glue language
A language such as Perl that is good at hooking things together that weren’t intended to be hooked
together.
granularity
The size of the pieces you’re dealing with, mentally speaking.
grapheme
A graphene is an allotrope of carbon arranged in a hexagonal crystal lattice one atom thick. A
grapheme, or more fully, a grapheme cluster string is a single user-visible character, which may in
turn be several characters (codepoints) long. For example, a carriage return plus a line feed is a
single grapheme but two characters, while a “ȫ” is a single grapheme but one, two, or even three
characters, depending on normalization.
greedy
A subpattern whose quantifier wants to match as many things as possible.
grep
Originally from the old Unix editor command for “Globally search for a Regular Expression and Print
it”, now used in the general sense of any kind of search, especially text searches. Perl has a built-
in "grep" function that searches a list for elements matching any given criterion, whereas the
grep(1) program searches for lines matching a regular expression in one or more files.
group
A set of users of which you are a member. In some operating systems (like Unix), you can give certain
file access permissions to other members of your group.
GV An internal “glob value” typedef, holding a typeglob. The "GV" type is a subclass of SV.
H
hacker
Someone who is brilliantly persistent in solving technical problems, whether these involve golfing,
fighting orcs, or programming. Hacker is a neutral term, morally speaking. Good hackers are not to
be confused with evil crackers or clueless script kiddies. If you confuse them, we will presume that
you are either evil or clueless.
handler
A subroutine or method that Perl calls when your program needs to respond to some internal event,
such as a signal, or an encounter with an operator subject to operator overloading. See also
callback.
hard reference
A scalar value containing the actual address of a referent, such that the referent’s reference count
accounts for it. (Some hard references are held internally, such as the implicit reference from one
of a typeglob’s variable slots to its corresponding referent.) A hard reference is different from a
symbolic reference.
hash
An unordered association of key/value pairs, stored such that you can easily use a string key to look
up its associated data value. This glossary is like a hash, where the word to be defined is the key
and the definition is the value. A hash is also sometimes septisyllabically called an “associative
array”, which is a pretty good reason for simply calling it a “hash” instead.
hash table
A data structure used internally by Perl for implementing associative arrays (hashes) efficiently.
See also bucket.
header file
A file containing certain required definitions that you must include “ahead” of the rest of your
program to do certain obscure operations. A C header file has a .h extension. Perl doesn’t really
have header files, though historically Perl has sometimes used translated .h files with a .ph
extension. See "require" in Camel chapter 27, “Functions”. (Header files have been superseded by the
module mechanism.)
here document
So called because of a similar construct in shells that pretends that the lines following the command
are a separate file to be fed to the command, up to some terminating string. In Perl, however, it’s
just a fancy form of quoting.
hexadecimal
A number in base 16, “hex” for short. The digits for 10 through 15 are customarily represented by the
letters "a" through "f". Hexadecimal constants in Perl start with "0x". See also the "hex" function
in Camel chapter 27, “Functions”.
home directory
The directory you are put into when you log in. On a Unix system, the name is often placed into
$ENV{HOME} or $ENV{LOGDIR} by login, but you can also find it with "(get""pwuid($<))[7]". (Some
platforms do not have a concept of a home directory.)
host
The computer on which a program or other data resides.
hubris
Excessive pride, the sort of thing for which Zeus zaps you. Also the quality that makes you write
(and maintain) programs that other people won’t want to say bad things about. Hence, the third great
virtue of a programmer. See also laziness and impatience.
HV Short for a “hash value” typedef, which holds Perl’s internal representation of a hash. The "HV" type
is a subclass of SV.
I
identifier
A legally formed name for most anything in which a computer program might be interested. Many
languages (including Perl) allow identifiers to start with an alphabetic character, and then contain
alphabetics and digits. Perl also allows connector punctuation like the underscore character wherever
it allows alphabetics. (Perl also has more complicated names, like qualified names.)
impatience
The anger you feel when the computer is being lazy. This makes you write programs that don’t just
react to your needs, but actually anticipate them. Or at least that pretend to. Hence, the second
great virtue of a programmer. See also laziness and hubris.
implementation
How a piece of code actually goes about doing its job. Users of the code should not count on
implementation details staying the same unless they are part of the published interface.
import
To gain access to symbols that are exported from another module. See "use" in Camel chapter 27,
“Functions”.
increment
To increase the value of something by 1 (or by some other number, if so specified).
indexing
In olden days, the act of looking up a key in an actual index (such as a phone book). But now it's
merely the act of using any kind of key or position to find the corresponding value, even if no index
is involved. Things have degenerated to the point that Perl’s "index" function merely locates the
position (index) of one string in another.
indirect filehandle
An expression that evaluates to something that can be used as a filehandle: a string (filehandle
name), a typeglob, a typeglob reference, or a low-level IO object.
indirection
If something in a program isn’t the value you’re looking for but indicates where the value is, that’s
indirection. This can be done with either symbolic references or hard.
indirect object
In English grammar, a short noun phrase between a verb and its direct object indicating the
beneficiary or recipient of the action. In Perl, "print STDOUT "$foo\n";" can be understood as “verb
indirect-object object”, where "STDOUT" is the recipient of the "print" action, and "$foo" is the
object being printed. Similarly, when invoking a method, you might place the invocant in the dative
slot between the method and its arguments:
$gollum = new Pathetic::Creature "Sméagol";
give $gollum "Fisssssh!";
give $gollum "Precious!";
indirect object slot
The syntactic position falling between a method call and its arguments when using the indirect object
invocation syntax. (The slot is distinguished by the absence of a comma between it and the next
argument.) "STDERR" is in the indirect object slot here:
print STDERR "Awake! Awake! Fear, Fire, Foes! Awake!\n";
infix
An operator that comes in between its operands, such as multiplication in "24 * 7".
inheritance
What you get from your ancestors, genetically or otherwise. If you happen to be a class, your
ancestors are called base classes and your descendants are called derived classes. See single
inheritance and multiple inheritance.
instance
Short for “an instance of a class”, meaning an object of that class.
instance data
See instance variable.
instance method
A method of an object, as opposed to a class method.
A method whose invocant is an object, not a package name. Every object of a class shares all the
methods of that class, so an instance method applies to all instances of the class, rather than
applying to a particular instance. Also see class method.
instance variable
An attribute of an object; data stored with the particular object rather than with the class as a
whole.
integer
A number with no fractional (decimal) part. A counting number, like 1, 2, 3, and so on, but including
0 and the negatives.
interface
The services a piece of code promises to provide forever, in contrast to its implementation, which it
should feel free to change whenever it likes.
interpolation
The insertion of a scalar or list value somewhere in the middle of another value, such that it
appears to have been there all along. In Perl, variable interpolation happens in double-quoted
strings and patterns, and list interpolation occurs when constructing the list of values to pass to a
list operator or other such construct that takes a "LIST".
interpreter
Strictly speaking, a program that reads a second program and does what the second program says
directly without turning the program into a different form first, which is what compilers do. Perl is
not an interpreter by this definition, because it contains a kind of compiler that takes a program
and turns it into a more executable form (syntax trees) within the perl process itself, which the
Perl runtime system then interprets.
invocant
The agent on whose behalf a method is invoked. In a class method, the invocant is a package name. In
an instance method, the invocant is an object reference.
invocation
The act of calling up a deity, daemon, program, method, subroutine, or function to get it to do what
you think it’s supposed to do. We usually “call” subroutines but “invoke” methods, since it sounds
cooler.
I/O Input from, or output to, a file or device.
IO An internal I/O object. Can also mean indirect object.
I/O layer
One of the filters between the data and what you get as input or what you end up with as output.
IPA India Pale Ale. Also the International Phonetic Alphabet, the standard alphabet used for phonetic
notation worldwide. Draws heavily on Unicode, including many combining characters.
IP Internet Protocol, or Intellectual Property.
IPC Interprocess Communication.
is-a
A relationship between two objects in which one object is considered to be a more specific version of
the other, generic object: “A camel is a mammal.” Since the generic object really only exists in a
Platonic sense, we usually add a little abstraction to the notion of objects and think of the
relationship as being between a generic base class and a specific derived class. Oddly enough,
Platonic classes don’t always have Platonic relationships—see inheritance.
iteration
Doing something repeatedly.
iterator
A special programming gizmo that keeps track of where you are in something that you’re trying to
iterate over. The "foreach" loop in Perl contains an iterator; so does a hash, allowing you to "each"
through it.
IV The integer four, not to be confused with six, Tom’s favorite editor. IV also means an internal
Integer Value of the type a scalar can hold, not to be confused with an NV.
J
JAPH
“Just Another Perl Hacker”, a clever but cryptic bit of Perl code that, when executed, evaluates to
that string. Often used to illustrate a particular Perl feature, and something of an ongoing
Obfuscated Perl Contest seen in USENET signatures.
K
key The string index to a hash, used to look up the value associated with that key.
keyword
See reserved words.
L
label
A name you give to a statement so that you can talk about that statement elsewhere in the program.
laziness
The quality that makes you go to great effort to reduce overall energy expenditure. It makes you
write labor-saving programs that other people will find useful, and then document what you wrote so
you don’t have to answer so many questions about it. Hence, the first great virtue of a programmer.
Also hence, this book. See also impatience and hubris.
leftmost longest
The preference of the regular expression engine to match the leftmost occurrence of a pattern, then
given a position at which a match will occur, the preference for the longest match (presuming the use
of a greedy quantifier). See Camel chapter 5, “Pattern Matching” for much more on this subject.
left shift
A bit shift that multiplies the number by some power of 2.
lexeme
Fancy term for a token.
lexer
Fancy term for a tokener.
lexical analysis
Fancy term for tokenizing.
lexical scoping
Looking at your Oxford English Dictionary through a microscope. (Also known as static scoping,
because dictionaries don’t change very fast.) Similarly, looking at variables stored in a private
dictionary (namespace) for each scope, which are visible only from their point of declaration down to
the end of the lexical scope in which they are declared. —Syn. static scoping. —Ant. dynamic
scoping.
lexical variable
A variable subject to lexical scoping, declared by "my". Often just called a “lexical”. (The "our"
declaration declares a lexically scoped name for a global variable, which is not itself a lexical
variable.)
library
Generally, a collection of procedures. In ancient days, referred to a collection of subroutines in a
.pl file. In modern times, refers more often to the entire collection of Perl modules on your system.
LIFO
Last In, First Out. See also FIFO. A LIFO is usually called a stack.
line
In Unix, a sequence of zero or more nonnewline characters terminated with a newline character. On
non-Unix machines, this is emulated by the C library even if the underlying operating system has
different ideas.
linebreak
A grapheme consisting of either a carriage return followed by a line feed or any character with the
Unicode Vertical Space character property.
line buffering
Used by a standard I/O output stream that flushes its buffer after every newline. Many standard I/O
libraries automatically set up line buffering on output that is going to the terminal.
line number
The number of lines read previous to this one, plus 1. Perl keeps a separate line number for each
source or input file it opens. The current source file’s line number is represented by "__LINE__".
The current input line number (for the file that was most recently read via "<FH>") is represented by
the $. ($INPUT_LINE_NUMBER) variable. Many error messages report both values, if available.
link
Used as a noun, a name in a directory that represents a file. A given file can have multiple links to
it. It’s like having the same phone number listed in the phone directory under different names. As a
verb, to resolve a partially compiled file’s unresolved symbols into a (nearly) executable image.
Linking can generally be static or dynamic, which has nothing to do with static or dynamic scoping.
LIST
A syntactic construct representing a comma- separated list of expressions, evaluated to produce a
list value. Each expression in a "LIST" is evaluated in list context and interpolated into the list
value.
list
An ordered set of scalar values.
list context
The situation in which an expression is expected by its surroundings (the code calling it) to return
a list of values rather than a single value. Functions that want a "LIST" of arguments tell those
arguments that they should produce a list value. See also context.
list operator
An operator that does something with a list of values, such as "join" or "grep". Usually used for
named built-in operators (such as "print", "unlink", and "system") that do not require parentheses
around their argument list.
list value
An unnamed list of temporary scalar values that may be passed around within a program from any list-
generating function to any function or construct that provides a list context.
literal
A token in a programming language, such as a number or string, that gives you an actual value instead
of merely representing possible values as a variable does.
little-endian
From Swift: someone who eats eggs little end first. Also used of computers that store the least
significant byte of a word at a lower byte address than the most significant byte. Often considered
superior to big-endian machines. See also big-endian.
local
Not meaning the same thing everywhere. A global variable in Perl can be localized inside a dynamic
scope via the "local" operator.
logical operator
Symbols representing the concepts “and”, “or”, “xor”, and “not”.
lookahead
An assertion that peeks at the string to the right of the current match location.
lookbehind
An assertion that peeks at the string to the left of the current match location.
loop
A construct that performs something repeatedly, like a roller coaster.
loop control statement
Any statement within the body of a loop that can make a loop prematurely stop looping or skip an
iteration. Generally, you shouldn’t try this on roller coasters.
loop label
A kind of key or name attached to a loop (or roller coaster) so that loop control statements can talk
about which loop they want to control.
lowercase
In Unicode, not just characters with the General Category of Lowercase Letter, but any character with
the Lowercase property, including Modifier Letters, Letter Numbers, some Other Symbols, and one
Combining Mark.
lvaluable
Able to serve as an lvalue.
lvalue
Term used by language lawyers for a storage location you can assign a new value to, such as a
variable or an element of an array. The “l” is short for “left”, as in the left side of an
assignment, a typical place for lvalues. An lvaluable function or expression is one to which a value
may be assigned, as in "pos($x) = 10".
lvalue modifier
An adjectival pseudofunction that warps the meaning of an lvalue in some declarative fashion.
Currently there are three lvalue modifiers: "my", "our", and "local".
M
magic
Technically speaking, any extra semantics attached to a variable such as $!, $0, %ENV, or %SIG, or to
any tied variable. Magical things happen when you diddle those variables.
magical increment
An increment operator that knows how to bump up ASCII alphabetics as well as numbers.
magical variables
Special variables that have side effects when you access them or assign to them. For example, in
Perl, changing elements of the %ENV array also changes the corresponding environment variables that
subprocesses will use. Reading the $! variable gives you the current system error number or message.
Makefile
A file that controls the compilation of a program. Perl programs don’t usually need a Makefile
because the Perl compiler has plenty of self-control.
man The Unix program that displays online documentation (manual pages) for you.
manpage
A “page” from the manuals, typically accessed via the man(1) command. A manpage contains a SYNOPSIS,
a DESCRIPTION, a list of BUGS, and so on, and is typically longer than a page. There are manpages
documenting commands, syscalls, library functions, devices, protocols, files, and such. In this book,
we call any piece of standard Perl documentation (like perlop or perldelta) a manpage, no matter what
format it’s installed in on your system.
matching
See pattern matching.
member data
See instance variable.
memory
This always means your main memory, not your disk. Clouding the issue is the fact that your machine
may implement virtual memory; that is, it will pretend that it has more memory than it really does,
and it’ll use disk space to hold inactive bits. This can make it seem like you have a little more
memory than you really do, but it’s not a substitute for real memory. The best thing that can be said
about virtual memory is that it lets your performance degrade gradually rather than suddenly when you
run out of real memory. But your program can die when you run out of virtual memory, too—if you
haven’t thrashed your disk to death first.
metacharacter
A character that is not supposed to be treated normally. Which characters are to be treated specially
as metacharacters varies greatly from context to context. Your shell will have certain
metacharacters, double-quoted Perl strings have other metacharacters, and regular expression patterns
have all the double-quote metacharacters plus some extra ones of their own.
metasymbol
Something we’d call a metacharacter except that it’s a sequence of more than one character.
Generally, the first character in the sequence must be a true metacharacter to get the other
characters in the metasymbol to misbehave along with it.
method
A kind of action that an object can take if you tell it to. See Camel chapter 12, “Objects”.
method resolution order
The path Perl takes through @INC. By default, this is a double depth first search, once looking for
defined methods and once for "AUTOLOAD". However, Perl lets you configure this with "mro".
minicpan
A CPAN mirror that includes just the latest versions for each distribution, probably created with
"CPAN::Mini". See Camel chapter 19, “CPAN”.
minimalism
The belief that “small is beautiful”. Paradoxically, if you say something in a small language, it
turns out big, and if you say it in a big language, it turns out small. Go figure.
mode
In the context of the stat(2) syscall, refers to the field holding the permission bits and the type
of the file.
modifier
See statement modifier, regular expression, and lvalue, not necessarily in that order.
module
A file that defines a package of (almost) the same name, which can either export symbols or function
as an object class. (A module’s main .pm file may also load in other files in support of the
module.) See the "use" built-in.
modulus
An integer divisor when you’re interested in the remainder instead of the quotient.
mojibake
When you speak one language and the computer thinks you’re speaking another. You’ll see odd
translations when you send UTF‑8, for instance, but the computer thinks you sent Latin-1, showing all
sorts of weird characters instead. The term is written 「文字化け」in Japanese and means “character
rot”, an apt description. Pronounced ["modʑibake"] in standard IPA phonetics, or approximately
“moh-jee-bah-keh”.
monger
Short for one member of Perl mongers, a purveyor of Perl.
mortal
A temporary value scheduled to die when the current statement finishes.
mro See method resolution order.
multidimensional array
An array with multiple subscripts for finding a single element. Perl implements these using
references—see Camel chapter 9, “Data Structures”.
multiple inheritance
The features you got from your mother and father, mixed together unpredictably. (See also inheritance
and single inheritance.) In computer languages (including Perl), it is the notion that a given class
may have multiple direct ancestors or base classes.
N
named pipe
A pipe with a name embedded in the filesystem so that it can be accessed by two unrelated processes.
namespace
A domain of names. You needn’t worry about whether the names in one such domain have been used in
another. See package.
NaN Not a number. The value Perl uses for certain invalid or inexpressible floating-point operations.
network address
The most important attribute of a socket, like your telephone’s telephone number. Typically an IP
address. See also port.
newline
A single character that represents the end of a line, with the ASCII value of 012 octal under Unix
(but 015 on a Mac), and represented by "\n" in Perl strings. For Windows machines writing text files,
and for certain physical devices like terminals, the single newline gets automatically translated by
your C library into a line feed and a carriage return, but normally, no translation is done.
NFS Network File System, which allows you to mount a remote filesystem as if it were local.
normalization
Converting a text string into an alternate but equivalent canonical (or compatible) representation
that can then be compared for equivalence. Unicode recognizes four different normalization forms:
NFD, NFC, NFKD, and NFKC.
null character
A character with the numeric value of zero. It’s used by C to terminate strings, but Perl allows
strings to contain a null.
null list
A list value with zero elements, represented in Perl by "()".
null string
A string containing no characters, not to be confused with a string containing a null character,
which has a positive length and is true.
numeric context
The situation in which an expression is expected by its surroundings (the code calling it) to return
a number. See also context and string context.
numification
(Sometimes spelled nummification and nummify.) Perl lingo for implicit conversion into a number; the
related verb is numify. Numification is intended to rhyme with mummification, and numify with
mummify. It is unrelated to English numen, numina, numinous. We originally forgot the extra m a long
time ago, and some people got used to our funny spelling, and so just as with "HTTP_REFERER"’s own
missing letter, our weird spelling has stuck around.
NV Short for Nevada, no part of which will ever be confused with civilization. NV also means an internal
floating- point Numeric Value of the type a scalar can hold, not to be confused with an IV.
nybble
Half a byte, equivalent to one hexadecimal digit, and worth four bits.
O
object
An instance of a class. Something that “knows” what user-defined type (class) it is, and what it can
do because of what class it is. Your program can request an object to do things, but the object gets
to decide whether it wants to do them or not. Some objects are more accommodating than others.
octal
A number in base 8. Only the digits 0 through 7 are allowed. Octal constants in Perl start with 0, as
in 013. See also the "oct" function.
offset
How many things you have to skip over when moving from the beginning of a string or array to a
specific position within it. Thus, the minimum offset is zero, not one, because you don’t skip
anything to get to the first item.
one-liner
An entire computer program crammed into one line of text.
open source software
Programs for which the source code is freely available and freely redistributable, with no commercial
strings attached. For a more detailed definition, see <http://www.opensource.org/osd.html>.
operand
An expression that yields a value that an operator operates on. See also precedence.
operating system
A special program that runs on the bare machine and hides the gory details of managing processes and
devices. Usually used in a looser sense to indicate a particular culture of programming. The loose
sense can be used at varying levels of specificity. At one extreme, you might say that all versions
of Unix and Unix-lookalikes are the same operating system (upsetting many people, especially lawyers
and other advocates). At the other extreme, you could say this particular version of this particular
vendor’s operating system is different from any other version of this or any other vendor’s operating
system. Perl is much more portable across operating systems than many other languages. See also
architecture and platform.
operator
A gizmo that transforms some number of input values to some number of output values, often built into
a language with a special syntax or symbol. A given operator may have specific expectations about
what types of data you give as its arguments (operands) and what type of data you want back from it.
operator overloading
A kind of overloading that you can do on built-in operators to make them work on objects as if the
objects were ordinary scalar values, but with the actual semantics supplied by the object class. This
is set up with the overload pragma—see Camel chapter 13, “Overloading”.
options
See either switches or regular expression modifiers.
ordinal
An abstract character’s integer value. Same thing as codepoint.
overloading
Giving additional meanings to a symbol or construct. Actually, all languages do overloading to one
extent or another, since people are good at figuring out things from context.
overriding
Hiding or invalidating some other definition of the same name. (Not to be confused with overloading,
which adds definitions that must be disambiguated some other way.) To confuse the issue further, we
use the word with two overloaded definitions: to describe how you can define your own subroutine to
hide a built-in function of the same name (see the section “Overriding Built-in Functions” in Camel
chapter 11, “Modules”), and to describe how you can define a replacement method in a derived class to
hide a base class’s method of the same name (see Camel chapter 12, “Objects”).
owner
The one user (apart from the superuser) who has absolute control over a file. A file may also have a
group of users who may exercise joint ownership if the real owner permits it. See permission bits.
P
package
A namespace for global variables, subroutines, and the like, such that they can be kept separate from
like-named symbols in other namespaces. In a sense, only the package is global, since the symbols in
the package’s symbol table are only accessible from code compiled outside the package by naming the
package. But in another sense, all package symbols are also globals—they’re just well-organized
globals.
pad Short for scratchpad.
parameter
See argument.
parent class
See base class.
parse tree
See syntax tree.
parsing
The subtle but sometimes brutal art of attempting to turn your possibly malformed program into a
valid syntax tree.
patch
To fix by applying one, as it were. In the realm of hackerdom, a listing of the differences between
two versions of a program as might be applied by the patch(1) program when you want to fix a bug or
upgrade your old version.
PATH
The list of directories the system searches to find a program you want to execute. The list is
stored as one of your environment variables, accessible in Perl as $ENV{PATH}.
pathname
A fully qualified filename such as /usr/bin/perl. Sometimes confused with "PATH".
pattern
A template used in pattern matching.
pattern matching
Taking a pattern, usually a regular expression, and trying the pattern various ways on a string to
see whether there’s any way to make it fit. Often used to pick interesting tidbits out of a file.
PAUSE
The Perl Authors Upload SErver (<http://pause.perl.org>), the gateway for modules on their way to
CPAN.
Perl mongers
A Perl user group, taking the form of its name from the New York Perl mongers, the first Perl user
group. Find one near you at <http://www.pm.org>.
permission bits
Bits that the owner of a file sets or unsets to allow or disallow access to other people. These flag
bits are part of the mode word returned by the "stat" built-in when you ask about a file. On Unix
systems, you can check the ls(1) manpage for more information.
Pern
What you get when you do "Perl++" twice. Doing it only once will curl your hair. You have to
increment it eight times to shampoo your hair. Lather, rinse, iterate.
pipe
A direct connection that carries the output of one process to the input of another without an
intermediate temporary file. Once the pipe is set up, the two processes in question can read and
write as if they were talking to a normal file, with some caveats.
pipeline
A series of processes all in a row, linked by pipes, where each passes its output stream to the next.
platform
The entire hardware and software context in which a program runs. A program written in a platform-
dependent language might break if you change any of the following: machine, operating system,
libraries, compiler, or system configuration. The perl interpreter has to be compiled differently for
each platform because it is implemented in C, but programs written in the Perl language are largely
platform independent.
pod The markup used to embed documentation into your Perl code. Pod stands for “Plain old documentation”.
See Camel chapter 23, “Plain Old Documentation”.
pod command
A sequence, such as "=head1", that denotes the start of a pod section.
pointer
A variable in a language like C that contains the exact memory location of some other item. Perl
handles pointers internally so you don’t have to worry about them. Instead, you just use symbolic
pointers in the form of keys and variable names, or hard references, which aren’t pointers (but act
like pointers and do in fact contain pointers).
polymorphism
The notion that you can tell an object to do something generic, and the object will interpret the
command in different ways depending on its type. [< Greek πολυ- + μορϕή, many forms.]
port
The part of the address of a TCP or UDP socket that directs packets to the correct process after
finding the right machine, something like the phone extension you give when you reach the company
operator. Also the result of converting code to run on a different platform than originally intended,
or the verb denoting this conversion.
portable
Once upon a time, C code compilable under both BSD and SysV. In general, code that can be easily
converted to run on another platform, where “easily” can be defined however you like, and usually is.
Anything may be considered portable if you try hard enough, such as a mobile home or London Bridge.
porter
Someone who “carries” software from one platform to another. Porting programs written in platform-
dependent languages such as C can be difficult work, but porting programs like Perl is very much
worth the agony.
possessive
Said of quantifiers and groups in patterns that refuse to give up anything once they’ve gotten their
mitts on it. Catchier and easier to say than the even more formal nonbacktrackable.
POSIX
The Portable Operating System Interface specification.
postfix
An operator that follows its operand, as in "$x++".
pp An internal shorthand for a “push- pop” code; that is, C code implementing Perl’s stack machine.
pragma
A standard module whose practical hints and suggestions are received (and possibly ignored) at
compile time. Pragmas are named in all lowercase.
precedence
The rules of conduct that, in the absence of other guidance, determine what should happen first. For
example, in the absence of parentheses, you always do multiplication before addition.
prefix
An operator that precedes its operand, as in "++$x".
preprocessing
What some helper process did to transform the incoming data into a form more suitable for the current
process. Often done with an incoming pipe. See also C preprocessor.
primary maintainer
The author that PAUSE allows to assign co-maintainer permissions to a namespace. A primary maintainer
can give up this distinction by assigning it to another PAUSE author. See Camel chapter 19, “CPAN”.
procedure
A subroutine.
process
An instance of a running program. Under multitasking systems like Unix, two or more separate
processes could be running the same program independently at the same time—in fact, the "fork"
function is designed to bring about this happy state of affairs. Under other operating systems,
processes are sometimes called “threads”, “tasks”, or “jobs”, often with slight nuances in meaning.
program
See script.
program generator
A system that algorithmically writes code for you in a high-level language. See also code generator.
progressive matching
Pattern matching matching>that picks up where it left off before.
property
See either instance variable or character property.
protocol
In networking, an agreed-upon way of sending messages back and forth so that neither correspondent
will get too confused.
prototype
An optional part of a subroutine declaration telling the Perl compiler how many and what flavor of
arguments may be passed as actual arguments, so you can write subroutine calls that parse much like
built-in functions. (Or don’t parse, as the case may be.)
pseudofunction
A construct that sometimes looks like a function but really isn’t. Usually reserved for lvalue
modifiers like "my", for context modifiers like "scalar", and for the pick-your-own-quotes
constructs, "q//", "qq//", "qx//", "qw//", "qr//", "m//", "s///", "y///", and "tr///".
pseudohash
Formerly, a reference to an array whose initial element happens to hold a reference to a hash. You
used to be able to treat a pseudohash reference as either an array reference or a hash reference.
Pseudohashes are no longer supported.
pseudoliteral
An operator X"that looks something like a literal, such as the output-grabbing operator, <literal
moreinfo="none""`>"command""`".
public domain
Something not owned by anybody. Perl is copyrighted and is thus not in the public domain—it’s just
freely available and freely redistributable.
pumpkin
A notional “baton” handed around the Perl community indicating who is the lead integrator in some
arena of development.
pumpking
A pumpkin holder, the person in charge of pumping the pump, or at least priming it. Must be willing
to play the part of the Great Pumpkin now and then.
PV A “pointer value”, which is Perl Internals Talk for a "char*".
Q
qualified
Possessing a complete name. The symbol $Ent::moot is qualified; $moot is unqualified. A fully
qualified filename is specified from the top-level directory.
quantifier
A component of a regular expression specifying how many times the foregoing atom may occur.
R
race condition
A race condition exists when the result of several interrelated events depends on the ordering of
those events, but that order cannot be guaranteed due to nondeterministic timing effects. If two or
more programs, or parts of the same program, try to go through the same series of events, one might
interrupt the work of the other. This is a good way to find an exploit.
readable
With respect to files, one that has the proper permission bit set to let you access the file. With
respect to computer programs, one that’s written well enough that someone has a chance of figuring
out what it’s trying to do.
reaping
The last rites performed by a parent process on behalf of a deceased child process so that it doesn’t
remain a zombie. See the "wait" and "waitpid" function calls.
record
A set of related data values in a file or stream, often associated with a unique key field. In Unix,
often commensurate with a line, or a blank-line–terminated set of lines (a “paragraph”). Each line
of the /etc/passwd file is a record, keyed on login name, containing information about that user.
recursion
The art of defining something (at least partly) in terms of itself, which is a naughty no-no in
dictionaries but often works out okay in computer programs if you’re careful not to recurse forever
(which is like an infinite loop with more spectacular failure modes).
reference
Where you look to find a pointer to information somewhere else. (See indirection.) References come in
two flavors: symbolic references and hard references.
referent
Whatever a reference refers to, which may or may not have a name. Common types of referents include
scalars, arrays, hashes, and subroutines.
regex
See regular expression.
regular expression
A single entity with various interpretations, like an elephant. To a computer scientist, it’s a
grammar for a little language in which some strings are legal and others aren’t. To normal people,
it’s a pattern you can use to find what you’re looking for when it varies from case to case. Perl’s
regular expressions are far from regular in the theoretical sense, but in regular use they work quite
well. Here’s a regular expression: "/Oh s.*t./". This will match strings like “"Oh say can you see
by the dawn's early light"” and “"Oh sit!"”. See Camel chapter 5, “Pattern Matching”.
regular expression modifier
An option on a pattern or substitution, such as "/i" to render the pattern case- insensitive.
regular file
A file that’s not a directory, a device, a named pipe or socket, or a symbolic link. Perl uses the
"–f" file test operator to identify regular files. Sometimes called a “plain” file.
relational operator
An operator that says whether a particular ordering relationship is true about a pair of operands.
Perl has both numeric and string relational operators. See collating sequence.
reserved words
A word with a specific, built-in meaning to a compiler, such as "if" or "delete". In many languages
(not Perl), it’s illegal to use reserved words to name anything else. (Which is why they’re reserved,
after all.) In Perl, you just can’t use them to name labels or filehandles. Also called “keywords”.
return value
The value produced by a subroutine or expression when evaluated. In Perl, a return value may be
either a list or a scalar.
RFC Request For Comment, which despite the timid connotations is the name of a series of important
standards documents.
right shift
A bit shift that divides a number by some power of 2.
role
A name for a concrete set of behaviors. A role is a way to add behavior to a class without
inheritance.
root
The superuser ("UID" == 0). Also the top-level directory of the filesystem.
RTFM
What you are told when someone thinks you should Read The Fine Manual.
run phase
Any time after Perl starts running your main program. See also compile phase. Run phase is mostly
spent in runtime but may also be spent in compile time when "require", "do" "FILE", or "eval"
"STRING" operators are executed, or when a substitution uses the "/ee" modifier.
runtime
The time when Perl is actually doing what your code says to do, as opposed to the earlier period of
time when it was trying to figure out whether what you said made any sense whatsoever, which is
compile time.
runtime pattern
A pattern that contains one or more variables to be interpolated before parsing the pattern as a
regular expression, and that therefore cannot be analyzed at compile time, but must be reanalyzed
each time the pattern match operator is evaluated. Runtime patterns are useful but expensive.
RV A recreational vehicle, not to be confused with vehicular recreation. RV also means an internal
Reference Value of the type a scalar can hold. See also IV and NV if you’re not confused yet.
rvalue
A value that you might find on the right side of an assignment. See also lvalue.
S
sandbox
A walled off area that’s not supposed to affect beyond its walls. You let kids play in the sandbox
instead of running in the road. See Camel chapter 20, “Security”.
scalar
A simple, singular value; a number, string, or reference.
scalar context
The situation in which an expression is expected by its surroundings (the code calling it) to return
a single value rather than a list of values. See also context and list context. A scalar context
sometimes imposes additional constraints on the return value—see string context and numeric context.
Sometimes we talk about a Boolean context inside conditionals, but this imposes no additional
constraints, since any scalar value, whether numeric or string, is already true or false.
scalar literal
A number or quoted string—an actual value in the text of your program, as opposed to a variable.
scalar value
A value that happens to be a scalar as opposed to a list.
scalar variable
A variable prefixed with "$" that holds a single value.
scope
From how far away you can see a variable, looking through one. Perl has two visibility mechanisms. It
does dynamic scoping of "local" variables, meaning that the rest of the block, and any subroutines
that are called by the rest of the block, can see the variables that are local to the block. Perl
does lexical scoping of "my" variables, meaning that the rest of the block can see the variable, but
other subroutines called by the block cannot see the variable.
scratchpad
The area in which a particular invocation of a particular file or subroutine keeps some of its
temporary values, including any lexically scoped variables.
script
A text file that is a program intended to be executed directly rather than compiled to another form
of file before execution.
Also, in the context of Unicode, a writing system for a particular language or group of languages,
such as Greek, Bengali, or Tengwar.
script kiddie
A cracker who is not a hacker but knows just enough to run canned scripts. A cargo-cult programmer.
sed A venerable Stream EDitor from which Perl derives some of its ideas.
semaphore
A fancy kind of interlock that prevents multiple threads or processes from using up the same
resources simultaneously.
separator
A character or string that keeps two surrounding strings from being confused with each other. The
"split" function works on separators. Not to be confused with delimiters or terminators. The “or” in
the previous sentence separated the two alternatives.
serialization
Putting a fancy data structure into linear order so that it can be stored as a string in a disk file
or database, or sent through a pipe. Also called marshalling.
server
In networking, a process that either advertises a service or just hangs around at a known location
and waits for clients who need service to get in touch with it.
service
Something you do for someone else to make them happy, like giving them the time of day (or of their
life). On some machines, well-known services are listed by the "getservent" function.
setgid
Same as setuid, only having to do with giving away group privileges.
setuid
Said of a program that runs with the privileges of its owner rather than (as is usually the case) the
privileges of whoever is running it. Also describes the bit in the mode word (permission bits) that
controls the feature. This bit must be explicitly set by the owner to enable this feature, and the
program must be carefully written not to give away more privileges than it ought to.
shared memory
A piece of memory accessible by two different processes who otherwise would not see each other’s
memory.
shebang
Irish for the whole McGillicuddy. In Perl culture, a portmanteau of “sharp” and “bang”, meaning the
"#!" sequence that tells the system where to find the interpreter.
shell
A command-line interpreter. The program that interactively gives you a prompt, accepts one or more
lines of input, and executes the programs you mentioned, feeding each of them their proper arguments
and input data. Shells can also execute scripts containing such commands. Under Unix, typical shells
include the Bourne shell (/bin/sh), the C shell (/bin/csh), and the Korn shell (/bin/ksh). Perl is
not strictly a shell because it’s not interactive (although Perl programs can be interactive).
side effects
Something extra that happens when you evaluate an expression. Nowadays it can refer to almost
anything. For example, evaluating a simple assignment statement typically has the “side effect” of
assigning a value to a variable. (And you thought assigning the value was your primary intent in the
first place!) Likewise, assigning a value to the special variable $| ($AUTOFLUSH) has the side effect
of forcing a flush after every "write" or "print" on the currently selected filehandle.
sigil
A glyph used in magic. Or, for Perl, the symbol in front of a variable name, such as "$", "@", and
"%".
signal
A bolt out of the blue; that is, an event triggered by the operating system, probably when you’re
least expecting it.
signal handler
A subroutine that, instead of being content to be called in the normal fashion, sits around waiting
for a bolt out of the blue before it will deign to execute. Under Perl, bolts out of the blue are
called signals, and you send them with the "kill" built-in. See the %SIG hash in Camel chapter 25,
“Special Names” and the section “Signals” in Camel chapter 15, “Interprocess Communication”.
single inheritance
The features you got from your mother, if she told you that you don’t have a father. (See also
inheritance and multiple inheritance.) In computer languages, the idea that classes reproduce
asexually so that a given class can only have one direct ancestor or base class. Perl supplies no
such restriction, though you may certainly program Perl that way if you like.
slice
A selection of any number of elements from a list, array, or hash.
slurp
To read an entire file into a string in one operation.
socket
An endpoint for network communication among multiple processes that works much like a telephone or a
post office box. The most important thing about a socket is its network address (like a phone
number). Different kinds of sockets have different kinds of addresses—some look like filenames, and
some don’t.
soft reference
See symbolic reference.
source filter
A special kind of module that does preprocessing on your script just before it gets to the tokener.
stack
A device you can put things on the top of, and later take them back off in the opposite order in
which you put them on. See LIFO.
standard
Included in the official Perl distribution, as in a standard module, a standard tool, or a standard
Perl manpage.
standard error
The default output stream for nasty remarks that don’t belong in standard output. Represented within
a Perl program by the output> filehandle "STDERR". You can use this stream explicitly, but the "die"
and "warn" built-ins write to your standard error stream automatically (unless trapped or otherwise
intercepted).
standard input
The default input stream for your program, which if possible shouldn’t care where its data is coming
from. Represented within a Perl program by the filehandle "STDIN".
standard I/O
A standard C library for doing buffered input and output to the operating system. (The “standard” of
standard I/O is at most marginally related to the “standard” of standard input and output.) In
general, Perl relies on whatever implementation of standard I/O a given operating system supplies, so
the buffering characteristics of a Perl program on one machine may not exactly match those on another
machine. Normally this only influences efficiency, not semantics. If your standard I/O package is
doing block buffering and you want it to flush the buffer more often, just set the $| variable to a
true value.
Standard Library
Everything that comes with the official perl distribution. Some vendor versions of perl change their
distributions, leaving out some parts or including extras. See also dual-lived.
standard output
The default output stream for your program, which if possible shouldn’t care where its data is going.
Represented within a Perl program by the filehandle "STDOUT".
statement
A command to the computer about what to do next, like a step in a recipe: “Add marmalade to batter
and mix until mixed.” A statement is distinguished from a declaration, which doesn’t tell the
computer to do anything, but just to learn something.
statement modifier
A conditional or loop that you put after the statement instead of before, if you know what we mean.
static
Varying slowly compared to something else. (Unfortunately, everything is relatively stable compared
to something else, except for certain elementary particles, and we’re not so sure about them.) In
computers, where things are supposed to vary rapidly, “static” has a derogatory connotation,
indicating a slightly dysfunctional variable, subroutine, or method. In Perl culture, the word is
politely avoided.
If you’re a C or C++ programmer, you might be looking for Perl’s "state" keyword.
static method
No such thing. See class method.
static scoping
No such thing. See lexical scoping.
static variable
No such thing. Just use a lexical variable in a scope larger than your subroutine, or declare it with
"state" instead of with "my".
stat structure
A special internal spot in which Perl keeps the information about the last file on which you
requested information.
status
The value returned to the parent process when one of its child processes dies. This value is placed
in the special variable $?. Its upper eight bits are the exit status of the defunct process, and its
lower eight bits identify the signal (if any) that the process died from. On Unix systems, this
status value is the same as the status word returned by wait(2). See "system" in Camel chapter 27,
“Functions”.
STDERR
See standard error.
STDIN
See standard input.
STDIO
See standard I/O.
STDOUT
See standard output.
stream
A flow of data into or out of a process as a steady sequence of bytes or characters, without the
appearance of being broken up into packets. This is a kind of interface—the underlying implementation
may well break your data up into separate packets for delivery, but this is hidden from you.
string
A sequence of characters such as “He said !@#*&%@#*?!”. A string does not have to be entirely
printable.
string context
The situation in which an expression is expected by its surroundings (the code calling it) to return
a string. See also context and numeric context.
stringification
The process of producing a string representation of an abstract object.
struct
C keyword introducing a structure definition or name.
structure
See data structure.
subclass
See derived class.
subpattern
A component of a regular expression pattern.
subroutine
A named or otherwise accessible piece of program that can be invoked from elsewhere in the program in
order to accomplish some subgoal of the program. A subroutine is often parameterized to accomplish
different but related things depending on its input arguments. If the subroutine returns a meaningful
value, it is also called a function.
subscript
A value that indicates the position of a particular array element in an array.
substitution
Changing parts of a string via the "s///" operator. (We avoid use of this term to mean variable
interpolation.)
substring
A portion of a string, starting at a certain character position (offset) and proceeding for a certain
number of characters.
superclass
See base class.
superuser
The person whom the operating system will let do almost anything. Typically your system administrator
or someone pretending to be your system administrator. On Unix systems, the root user. On Windows
systems, usually the Administrator user.
SV Short for “scalar value”. But within the Perl interpreter, every referent is treated as a member of a
class derived from SV, in an object-oriented sort of way. Every value inside Perl is passed around as
a C language "SV*" pointer. The SV struct knows its own “referent type”, and the code is smart enough
(we hope) not to try to call a hash function on a subroutine.
switch
An option you give on a command line to influence the way your program works, usually introduced with
a minus sign. The word is also used as a nickname for a switch statement.
switch cluster
The combination of multiple command- line switches (e.g., "–a –b –c") into one switch (e.g., "–abc").
Any switch with an additional argument must be the last switch in a cluster.
switch statement
A program technique that lets you evaluate an expression and then, based on the value of the
expression, do a multiway branch to the appropriate piece of code for that value. Also called a “case
structure”, named after the similar Pascal construct. Most switch statements in Perl are spelled
"given". See “The "given" statement” in Camel chapter 4, “Statements and Declarations”.
symbol
Generally, any token or metasymbol. Often used more specifically to mean the sort of name you might
find in a symbol table.
symbolic debugger
A program that lets you step through the execution of your program, stopping or printing things out
here and there to see whether anything has gone wrong, and, if so, what. The “symbolic” part just
means that you can talk to the debugger using the same symbols with which your program is written.
symbolic link
An alternate filename that points to the real filename, which in turn points to the real file.
Whenever the operating system is trying to parse a pathname containing a symbolic link, it merely
substitutes the new name and continues parsing.
symbolic reference
A variable whose value is the name of another variable or subroutine. By dereferencing the first
variable, you can get at the second one. Symbolic references are illegal under "use strict "refs"".
symbol table
Where a compiler remembers symbols. A program like Perl must somehow remember all the names of all
the variables, filehandles, and subroutines you’ve used. It does this by placing the names in a
symbol table, which is implemented in Perl using a hash table. There is a separate symbol table for
each package to give each package its own namespace.
synchronous
Programming in which the orderly sequence of events can be determined; that is, when things happen
one after the other, not at the same time.
syntactic sugar
An alternative way of writing something more easily; a shortcut.
syntax
From Greek σύνταξις, “with-arrangement”. How things (particularly symbols) are put together with each
other.
syntax tree
An internal representation of your program wherein lower-level constructs dangle off the higher-level
constructs enclosing them.
syscall
A function call directly to the operating system. Many of the important subroutines and functions you
use aren’t direct system calls, but are built up in one or more layers above the system call level.
In general, Perl programmers don’t need to worry about the distinction. However, if you do happen to
know which Perl functions are really syscalls, you can predict which of these will set the $!
($ERRNO) variable on failure. Unfortunately, beginning programmers often confusingly employ the term
“system call” to mean what happens when you call the Perl "system" function, which actually involves
many syscalls. To avoid any confusion, we nearly always say “syscall” for something you could call
indirectly via Perl’s "syscall" function, and never for something you would call with Perl’s "system"
function.
T
taint checks
The special bookkeeping Perl does to track the flow of external data through your program and
disallow their use in system commands.
tainted
Said of data derived from the grubby hands of a user, and thus unsafe for a secure program to rely
on. Perl does taint checks if you run a setuid (or setgid) program, or if you use the "–T" switch.
taint mode
Running under the "–T" switch, marking all external data as suspect and refusing to use it with
system commands. See Camel chapter 20, “Security”.
TCP Short for Transmission Control Protocol. A protocol wrapped around the Internet Protocol to make an
unreliable packet transmission mechanism appear to the application program to be a reliable stream of
bytes. (Usually.)
term
Short for a “terminal”—that is, a leaf node of a syntax tree. A thing that functions grammatically as
an operand for the operators in an expression.
terminator
A character or string that marks the end of another string. The $/ variable contains the string that
terminates a "readline" operation, which "chomp" deletes from the end. Not to be confused with
delimiters or separators. The period at the end of this sentence is a terminator.
ternary
An operator taking three operands. Sometimes pronounced trinary.
text
A string or file containing primarily printable characters.
thread
Like a forked process, but without fork’s inherent memory protection. A thread is lighter weight than
a full process, in that a process could have multiple threads running around in it, all fighting over
the same process’s memory space unless steps are taken to protect threads from one another.
tie The bond between a magical variable and its implementation class. See the "tie" function in Camel
chapter 27, “Functions” and Camel chapter 14, “Tied Variables”.
titlecase
The case used for capitals that are followed by lowercase characters instead of by more capitals.
Sometimes called sentence case or headline case. English doesn’t use Unicode titlecase, but casing
rules for English titles are more complicated than simply capitalizing each word’s first character.
TMTOWTDI
There’s More Than One Way To Do It, the Perl Motto. The notion that there can be more than one valid
path to solving a programming problem in context. (This doesn’t mean that more ways are always better
or that all possible paths are equally desirable—just that there need not be One True Way.)
token
A morpheme in a programming language, the smallest unit of text with semantic significance.
tokener
A module that breaks a program text into a sequence of tokens for later analysis by a parser.
tokenizing
Splitting up a program text into tokens. Also known as “lexing”, in which case you get “lexemes”
instead of tokens.
toolbox approach
The notion that, with a complete set of simple tools that work well together, you can build almost
anything you want. Which is fine if you’re assembling a tricycle, but if you’re building a
defranishizing comboflux regurgalator, you really want your own machine shop in which to build
special tools. Perl is sort of a machine shop.
topic
The thing you’re working on. Structures like while(<>), "for", "foreach", and "given" set the topic
for you by assigning to $_, the default (topic) variable.
transliterate
To turn one string representation into another by mapping each character of the source string to its
corresponding character in the result string. Not to be confused with translation: for example, Greek
πολύχρωμος transliterates into polychromos but translates into many-colored. See the "tr///" operator
in Camel chapter 5, “Pattern Matching”.
trigger
An event that causes a handler to be run.
trinary
Not a stellar system with three stars, but an operator taking three operands. Sometimes pronounced
ternary.
troff
A venerable typesetting language from which Perl derives the name of its $% variable and which is
secretly used in the production of Camel books.
true
Any scalar value that doesn’t evaluate to 0 or "".
truncating
Emptying a file of existing contents, either automatically when opening a file for writing or
explicitly via the "truncate" function.
type
See data type and class.
type casting
Converting data from one type to another. C permits this. Perl does not need it. Nor want it.
typedef
A type definition in the C and C++ languages.
typed lexical
A lexical variable lexical>that is declared with a class type: "my Pony $bill".
typeglob
Use of a single identifier, prefixed with "*". For example, *name stands for any or all of $name,
@name, %name, &name, or just "name". How you use it determines whether it is interpreted as all or
only one of them. See “Typeglobs and Filehandles” in Camel chapter 2, “Bits and Pieces”.
typemap
A description of how C types may be transformed to and from Perl types within an extension module
written in XS.
U
UDP User Datagram Protocol, the typical way to send datagrams over the Internet.
UID A user ID. Often used in the context of file or process ownership.
umask
A mask of those permission bits that should be forced off when creating files or directories, in
order to establish a policy of whom you’ll ordinarily deny access to. See the "umask" function.
unary operator
An operator with only one operand, like "!" or "chdir". Unary operators are usually prefix operators;
that is, they precede their operand. The "++" and "––" operators can be either prefix or postfix.
(Their position does change their meanings.)
Unicode
A character set comprising all the major character sets of the world, more or less. See
<http://www.unicode.org>.
Unix
A very large and constantly evolving language with several alternative and largely incompatible
syntaxes, in which anyone can define anything any way they choose, and usually do. Speakers of this
language think it’s easy to learn because it’s so easily twisted to one’s own ends, but dialectical
differences make tribal intercommunication nearly impossible, and travelers are often reduced to a
pidgin-like subset of the language. To be universally understood, a Unix shell programmer must spend
years of study in the art. Many have abandoned this discipline and now communicate via an Esperanto-
like language called Perl.
In ancient times, Unix was also used to refer to some code that a couple of people at Bell Labs wrote
to make use of a PDP-7 computer that wasn’t doing much of anything else at the time.
uppercase
In Unicode, not just characters with the General Category of Uppercase Letter, but any character with
the Uppercase property, including some Letter Numbers and Symbols. Not to be confused with titlecase.
V
value
An actual piece of data, in contrast to all the variables, references, keys, indices, operators, and
whatnot that you need to access the value.
variable
A named storage location that can hold any of various kinds of value, as your program sees fit.
variable interpolation
The interpolation of a scalar or array variable into a string.
variadic
Said of a function that happily receives an indeterminate number of actual arguments.
vector
Mathematical jargon for a list of scalar values.
virtual
Providing the appearance of something without the reality, as in: virtual memory is not real memory.
(See also memory.) The opposite of “virtual” is “transparent”, which means providing the reality of
something without the appearance, as in: Perl handles the variable-length UTF‑8 character encoding
transparently.
void context
A form of scalar context in which an expression is not expected to return any value at all and is
evaluated for its side effects alone.
v-string
A “version” or “vector” string specified with a "v" followed by a series of decimal integers in dot
notation, for instance, "v1.20.300.4000". Each number turns into a character with the specified
ordinal value. (The "v" is optional when there are at least three integers.)
W
warning
A message printed to the "STDERR" stream to the effect that something might be wrong but isn’t worth
blowing up over. See "warn" in Camel chapter 27, “Functions” and the "warnings" pragma in Camel
chapter 28, “Pragmantic Modules”.
watch expression
An expression which, when its value changes, causes a breakpoint in the Perl debugger.
weak reference
A reference that doesn’t get counted normally. When all the normal references to data disappear, the
data disappears. These are useful for circular references that would never disappear otherwise.
whitespace
A character that moves your cursor but doesn’t otherwise put anything on your screen. Typically
refers to any of: space, tab, line feed, carriage return, or form feed. In Unicode, matches many
other characters that Unicode considers whitespace, including the ɴ-ʙʀ .
word
In normal “computerese”, the piece of data of the size most efficiently handled by your computer,
typically 32 bits or so, give or take a few powers of 2. In Perl culture, it more often refers to an
alphanumeric identifier (including underscores), or to a string of nonwhitespace characters bounded
by whitespace or string boundaries.
working directory
Your current directory, from which relative pathnames are interpreted by the operating system. The
operating system knows your current directory because you told it with a "chdir", or because you
started out in the place where your parent process was when you were born.
wrapper
A program or subroutine that runs some other program or subroutine for you, modifying some of its
input or output to better suit your purposes.
WYSIWYG
What You See Is What You Get. Usually used when something that appears on the screen matches how it
will eventually look, like Perl’s "format" declarations. Also used to mean the opposite of magic
because everything works exactly as it appears, as in the three- argument form of "open".
X
XS An extraordinarily exported, expeditiously excellent, expressly eXternal Subroutine, executed in
existing C or C++ or in an exciting extension language called (exasperatingly) XS.
XSUB
An external subroutine defined in XS.
Y
yacc
Yet Another Compiler Compiler. A parser generator without which Perl probably would not have existed.
See the file perly.y in the Perl source distribution.
Z
zero width
A subpattern assertion matching the null string between characters.
zombie
A process that has died (exited) but whose parent has not yet received proper notification of its
demise by virtue of having called "wait" or "waitpid". If you "fork", you must clean up after your
child processes when they exit; otherwise, the process table will fill up and your system
administrator will Not Be Happy with you.
AUTHOR AND COPYRIGHT
Based on the Glossary of Programming Perl, Fourth Edition, by Tom Christiansen, brian d foy, Larry Wall,
& Jon Orwant. Copyright (c) 2000, 1996, 1991, 2012 O'Reilly Media, Inc. This document may be
distributed under the same terms as Perl itself.
perl v5.38.2 2025-04-08 PERLGLOSSARY(1)