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For other uses, see Perl (disambiguation).
Perl

File:Programming-republic-of-perl.gif
Paradigmmulti-paradigm
Designed byLarry Wall
First appeared1987
Stable release
5.8.8 / January 31, 2006
Typing disciplinedynamic
OSCross-platform
LicenseGPL or Artistic License
Websitewww.perl.org
Influenced by
AWK, BASIC-PLUS, C, C++, Lisp, Pascal, Python, Sed, Unix shell
Influenced
Python, PHP, Ruby

Perl, also Practical Extraction and Report Language (a backronym, see below) is a dynamic procedural programming language designed by Larry Wall and first released in 1987. Perl borrows features from C, shell scripting (sh), AWK, sed, Lisp, and, to a lesser extent, many other programming languages.

Overview

The perlintro(1) man page states:

Perl is a general-purpose programming language originally developed for text manipulation and now used for a wide range of tasks including system administration, web development, network programming, GUI development, and more.
The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal). Its major features are that it's easy to use, supports both procedural and object-oriented (OO) programming, has powerful built-in support for text processing, and has a large collection of third-party modules.

Features

The overall structure of Perl derives broadly from the programming language C. Perl is a procedural programming language, with variables, expressions, assignment statements, brace-delimited code blocks, control structures, and subroutines.

Perl also takes features from shell programming. All variables are marked with leading sigils. Sigils unambiguously identify variable names, allowing Perl to have a rich syntax. Importantly, sigils allow variables to be interpolated directly into strings. Like the Unix shells, Perl has many built-in functions for common tasks, like sorting, and for accessing system facilities.

Perl takes lists from Lisp, associative arrays from AWK, and regular expressions from sed. These simplify and facilitate all manner of parsing, text handling, and data management tasks.

In Perl 5, features were added that support complex data structures, first-class functions (i.e. closures as values), and an object-oriented programming model. These include references, packages, and class-based method dispatch. Perl 5 also saw the introduction of lexically scoped variables, which make it easier to write robust code, and modules, which make it practical to write and distribute libraries of Perl code.

All versions of Perl do automatic data typing and memory management. The interpreter knows the type and storage requirements of every data object in the program; it allocates and frees storage for them as necessary. Legal type conversions are done automatically at run time; illegal type conversions are fatal errors.

Applications

Perl has many and varied applications.

It has been used since the early days of the Web to write CGI scripts, and is an integral component of the popular LAMP (Linux / Apache / MySQL / Perl, PHP, and Python) platform for web development. Large projects written in Perl include Slash, early implementations of PHP[1], and UseModWiki, the wiki software used in Wikipedia until 2002. It's known as one of "the three Ps" (Perl, Python, and PHP), which are the most popular server-side, open source scripting languages for the Web, though open source Java and C# implementations as well as Ruby have grown popular in recent years.

Perl is often used as a "glue language", tying together systems and interfaces that were not specifically designed to interoperate. Systems administrators use Perl as an all-purpose tool; short Perl programs can be entered and run on a single command line.

Perl is widely used in finance and bioinformatics, where it is valued for rapid application development, ability to handle large data sets, and the availability of many standard and third-party modules.

Implementation

Perl is implemented as a core interpreter, written in C, together with a large collection of modules, written in Perl and C. The source distribution is, as of 2005, 12 MB when packaged in a tar file and compressed. The interpreter is 150,000 lines of C code and compiles to a 1 MB executable on typical machine architectures. Alternatively, the interpreter can be compiled to a link library and embedded in other programs. There are nearly 500 modules in the distribution, comprising 200,000 lines of Perl and an additional 350,000 lines of C code. Much of the C code in the modules consists of character encoding tables.

The interpreter has an object-oriented architecture. All of the elements of the Perl language—scalars, arrays, hashes, coderefs, file handles—are represented in the interpreter by C structs. Operations on these structs are defined by a large collection of macros, typedefs and functions; these constitute the Perl C API. The Perl API can be bewildering to the uninitiated, but its entry points follow a consistent naming scheme, which provides guidance to those who use it.

The execution of a Perl program divides broadly into two phases: compile-time and run-time. At compile time, the interpreter parses the program text into a syntax tree. At run time, it executes the program by walking the tree. The text is parsed only once, and the syntax tree is subject to optimization before it is executed, so the execution phase is relatively efficient. Compile-time optimizations on the syntax tree include constant folding, context propagation, and peephole optimization.

Perl is a dynamic language and has a context-sensitive grammar that cannot be parsed by a straight Lex/Yacc lexer/parser combination. Instead, it implements its own lexer, which coordinates with a modified GNU bison parser to resolve ambiguities in the language. It is said that "only perl can parse Perl", meaning that only the Perl interpreter (perl) can parse the Perl language (Perl). The truth of this is attested to by the persistent imperfections of other programs that undertake to parse Perl, such as source code analyzers and auto-indenters.

Maintenance of the Perl interpreter has become increasingly difficult over the years. The code base has been in continuous development since 1994. The code has been optimized for performance at the expense of simplicity, clarity, and strong internal interfaces. New features have been added, yet virtually complete backward compatibility with earlier versions is maintained. The size and complexity of the interpreter is a barrier to developers who wish to work on it.

Perl is distributed with some 120,000 functional tests. These run as part of the normal build process, and extensively exercise the interpreter and its core modules. Perl developers rely on the functional tests to ensure that changes to the interpreter do not introduce bugs; conversely, Perl users who see the interpreter pass its functional tests on their system can have a high degree of confidence that it is working properly.

There is no written specification or standard for the Perl language, and no plans to create one for the current version of Perl. There has only ever been one implementation of the interpreter. That interpreter, together with its functional tests, stands as a de facto specification of the language.

Availability

Perl is free software, and is licensed under both the Artistic License and the GNU General Public License. Distributions are available for most operating systems. It is particularly prevalent on Unix and Unix-like systems (such as Linux, FreeBSD, and Mac OS X), and is available on Microsoft Windows systems.

Perl has been ported to over a hundred different platforms, and can, with only six reported exceptions, be compiled from source on all Unix-like, POSIX-compliant or otherwise Unix-compatible platforms, including AmigaOS, BeOS, Cygwin, and Mac OS X (See ports). A special port, MacPerl, is available for Mac OS Classic.

Perl can be compiled from source on Windows, however many Windows installations lack a C compiler, so Windows users typically install a binary distribution, such as ActivePerl, PXPerl or IndigoPerl. Perl can also be compiled with gcc under cygwin, which provides a unix-like environment on Windows.

In June 2006, Adam Kennedy on behalf of the The Perl Foundation launched win32.perl.org, a community website for "all things Windows and Perl".

Language structure

Example Program

In Perl, the canonical "Hello world" program is: 

#!/usr/bin/perl
print "Hello, world!\n";

The first line is the shebang, which tells the operating system where to find the Perl interpreter. The second line prints the string Hello, world! and a newline.

The shebang is the usual way to invoke the interpreter on Unix systems. Windows systems may rely on the shebang, or they may associate a .pl file extension with the Perl interpreter.

Data types

Perl has three fundamental data types: scalars, lists, and hashes:

  • A scalar is a single value; it may be a number, a string or a reference
  • A list is an ordered collection of scalars (a variable that holds a list is called an array)
  • A hash, or associative array, is a map from strings to scalars; the strings are called keys and the scalars are called values.

All variables are marked by a leading sigil, which identifies the data type. The same name may be used for variables of different types, without conflict. 

 $foo   # a scalar
 @foo   # a list
 %foo   # a hash

Numbers are written in the usual way; strings are enclosed by quotes of various kinds. 

 $n     = 42;
 $name  = "joe";
 $color = 'red';

Perl will decide if your string is a number or not based on the context in which it is used. It tries to convert strings into numbers when using them with numerical operators. In the following example the strings $n and $m are treated as numbers when they are the arguments to the addition operator. This code prints the number '5', discarding non number information for the operation, although the variable values remain the same. 

 $n     = "3 apples";
 $m     = "2 oranges";
 print $n + $m;

Perl also has a boolean context that it uses in evaluating conditional statements. The following values all evaluate as false in Perl: 

 $false = 0;     # the number zero
 $false = '0';   # the string zero
 $false = "";    # the empty string
 $false = undef; # the return value from undef

All other values are evaluated to true. This includes the odd self-describing string of "0 but true", which in fact is 0 as a number, but true when used as a boolean. (Any non-numeric string would also have this property, but this particular string is ignored by Perl with respect to numeric warnings.) Evaluated boolean expressions also return scalar values. Although the documentation does not promise which particular true or false is returned (and thus cannot be relied on), many boolean operators return 1 for true and the empty-string for false (which evaluates to zero in a numeric context). If you need to specifically ensure a 1/0 result (as in C), you need to do the math yourself: 

 my $real_result = $boolean_result ? 1 : 0;

A list is written by listing its elements, separated by commas, and enclosed by parentheses where required by operator precedence. 

 @scores = (32, 45, 16, 5);

A hash may be initialized from a list of key/value pairs. 

 %favorite = (joe => 'red',
              sam => 'blue');

Individual elements of a list are accessed by providing a numerical index, in square brackets. Individual values in a hash are accessed by providing the corresponding key, in curly braces. The $ sigil identifies the accessed element as a scalar. 

 $scores[2]      # an element of @scores
 $favorite{joe}  # a value in %favorite

Multiple elements may be accessed by using the @ sigil instead (identifying the result as a list). 

 @scores[2, 3, 1]    # three elements of @scores
 @favorite{'joe', 'sam'} # two values in %favorite

The number of elements in an array can be obtained by evaluating the array in scalar context or with the help of the $# sigil. The latter gives the index of the last element in the array, not the number of elements. 

 $count = @friends;
 $#friends       # the index of the last element in @friends
 $#friends+1     # usually the number of elements in @friends
                 # this is one more than $#friends because the first element is at
                 # index 0, not 1

There are a few functions that operate on entire hashes. 

 @names     = keys   %address;
 @addresses = values %address;

Control structures

Main article: Perl control structures

Perl has several kinds of control structures.

It has block-oriented control structures, similar to those in the C and Java programming languages. Conditions are surrounded by parentheses, and controlled blocks are surrounded by braces: 

label while ( cond ) { ... }
label while ( cond ) { ... } continue { ... }
label for ( init-expr ; cond-expr ; incr-expr ) { ... }
label foreach var ( list ) { ... }
label foreach var ( list ) { ... } continue { ... }
if ( cond ) { ... }
if ( cond ) { ... } else { ... } 
if ( cond ) { ... } elsif ( cond ) { ... } else { ... }

Where only a single statement is being controlled, statement modifiers provide a lighter syntax: 

statement if      cond ;
statement unless  cond ;
statement while   cond ;
statement until   cond ;
statement foreach list ;

Short-circuit logical operators are commonly used to effect control flow at the expression level: 

expr and expr
expr or  expr

The flow control keywords next, last, return, and redo are expressions, so they can be used with short-circuit operators.

Perl also has two implicit looping constructs: 

 results = grep { ... } list
 results = map  { ... } list

grep returns all elements of list for which the controlled block evaluates to true. map evaluates the controlled block for each element of list and returns a list of the resulting values. These constructs enable a simple functional programming style.

There is no switch statement (multi-way branch) in Perl 5. The Perl documentation describes a half-dozen ways to achieve the same effect by using other control structures. There is a Switch module, however, which provides functionality modeled on the forthcoming Perl 6 re-design.

Perl includes a goto label statement, but it is rarely used. Situations where a goto is called for in other languages don't occur as often in Perl due to its breadth of flow control options.

There is also a goto &sub statement that performs a tail call. It terminates the current subroutine and immediately calls the specified sub. This is used in situations where a caller can perform more efficient stack management than Perl itself (typically because no change to the current stack is required), and in deep recursion tail calling can have substantial positive impact on performance because it avoids the overhead of scope/stack management on return.

Subroutines

Subroutines are defined with the sub keyword, and invoked simply by naming them. Subroutine definitions may appear anywhere in the program. Parentheses are required for calls that precede the definition. 

foo();
sub foo { ... }
foo;

A list of arguments may be provided after the subroutine name. Arguments may be scalars, lists, or hashes. 

foo $x, @y, %z;

The parameters to a subroutine need not be declared as to either number or type; in fact, they may vary from call to call. Arrays are expanded to their elements, hashes are expanded to a list of key/value pairs, and the whole lot is passed into the subroutine as one undifferentiated list of scalars.

Whatever arguments are passed are available to the subroutine in the special array @_. The elements of @_ are aliased to the actual arguments; changing an element of @_ changes the corresponding argument.

Elements of @_ may be accessed by subscripting it in the usual way. 

$_[0], $_[1]

However, the resulting code can be difficult to read, and the parameters have pass-by-reference semantics, which may be undesirable.

One common idiom is to assign @_ to a list of named variables. 

my($x, $y, $z) = @_;

This effects both mnemonic parameter names and pass-by-value semantics. The my keyword indicates that the following variables are lexically scoped to the containing block.

Another idiom is to shift parameters off of @_. This is especially common when the subroutine takes only one argument. 

my $x = shift;

Subroutines may return values. 

return 42, $x, @y, %z;

If the subroutine does not exit via a return statement, then it returns the last expression evaluated within the subroutine body. Arrays and hashes in the return value are expanded to lists of scalars, just as they are for arguments.

The returned expression is evaluated in the calling context of the subroutine; this can surprise the unwary. 

sub list  {      (4, 5, 6)     }
sub array { @x = (4, 5, 6); @x }

$x = list;   # returns 6 - last element of list
$x = array;  # returns 3 - number of elements in list
@x = list;   # returns (4, 5, 6)
@x = array;  # returns (4, 5, 6)

A subroutine can discover its calling context with the wantarray function. 

sub either { wantarray ? (1, 2) : "Oranges" }

$x = either;    # returns "Oranges"
@x = either;    # returns (1, 2)

Regular expressions

See also: Perl regular expression examples and Regular expression

The Perl language includes a specialized syntax for writing regular expressions (REs), and the interpreter contains an engine for matching strings to regular expressions. The regular expression engine uses a backtracking algorithm, extending its capabilities from simple pattern matching to string capture and substitution. The regular expression engine is derived from regex written by Henry Spencer.

The Perl regular expression syntax was originally taken from Unix Version 8 regular expressions. However, it diverged before the first release of Perl, and has since grown to include many more features. Other languages and applications are now adopting Perl compatible regular expressions over POSIX regular expressions including PHP, Ruby, Java, and the Apache HTTP server.

The m// (match) operator introduces a regular expression match. (The leading m may be omitted for brevity.) In the simplest case, an expression like 

 $x =~ m/abc/

evaluates to true if and only if the string $x matches the regular expression abc.

Portions of a regular expression may be enclosed in parentheses; corresponding portions of a matching string are captured. Captured strings are assigned to the sequential built-in variables $1, $2, $3, ..., and a list of captured strings is returned as the value of the match. 

 $x =~ m/a(.)c/;  # capture the character between 'a' and 'c'

The s/// (substitute) operator specifies a search and replace operation: 

 $x =~ s/abc/aBc/;   # upcase the b

Perl regular expressions can take modifiers. These are single-letter suffixes that modify the meaning of the expression: 

 $x =~ m/abc/i;      # case-insensitive pattern match
 $x =~ s/abc/aBc/g;  # global search and replace

Regular expressions can be dense and cryptic. This is because regular expression syntax is extremely compact, generally using single characters or character pairs to represent its operations. Perl provides some relief from this problem with the /x modifer, which allows programmers to place whitespace and comments inside regular expressions: 

 $x =~ m/a     # match 'a'
         .     # match any character
         c     # match 'c'
          /x;

One common use of regular expressions is to specify delimiters for the split operator: 

 @words = split m/,/, $line;   # divide $line into comma-separated values

The split operator complements string capture. String capture returns the parts of a string that match a regular expression; split returns the parts that don't match.

Database interfaces

Perl is widely favored for database applications. Its text handling facilities are good for generating SQL queries; arrays, hashes and automatic memory management make it easy to collect and process the returned data.

In early versions of Perl, database interfaces were created by relinking the interpreter with a client-side database library. This was somewhat clumsy; a particular problem was that the resulting perl executable was restricted to using just the one database interface that it was linked to. Also, relinking the interpreter was sufficiently difficult that it was only done for a few of the most important and widely used databases.

In Perl 5, database interfaces are implemented by Perl DBI modules. The DBI (Database Interface) module presents a single, database-independent interface to Perl applications, while the DBD:: (Database Driver) modules handle the details of accessing some 50 different databases. There are DBD:: drivers for most ANSI SQL databases.

Comparative performance

Benchmarks that compare Perl to other languages are often quite difficult, as working code and the most efficient code are not always the same thing (a result of the "there's more than one way to do it" philosophy). Combined with the fact that benchmarks almost always favor the languages which the benchmark authors understand the most, this leads to a strong skepticism of such measurements.

The "Computer Language Shootout Benchmarks" [2] compares the performance of implementations of typical programming problems in several programming languages. Their Perl implementations typically took up more memory than implementations in other languages, and had varied speed results. Nicholas Clark, a Perl core developer, discusses some of Perl design trade-offs and why Perl may be slower (along with some solutions) in "When perl is not quite fast enough" [3].

Perl may be slower than other languages doing the same thing because it has to compile the source every time it runs. In "A Timely Start" [4], Jean-Louis Leroy found that his Perl scripts took much longer to run than he expected because the perl interpreter spent much of the time finding and compiling modules. Since Perl cannot save its intermediate result like Java, Python, and Ruby can, Perl scripts pay this overhead penalty on every execution. Once perl starts the run phase, however, it can be quite fast. The overhead is not such a problem when amortized over a long run phase, but can be a significant performance limiter for very short execution times (as often found in benchmarks).

Optimizing Perl can require intimate knowledge of its workings rather than skill with the language and its syntax, meaning that the problem is with the implementation of Perl rather than the language itself. Perl 6, the next major version, will address some of these lessons that other languages have already learned.

Origins

History

Larry Wall began work on Perl in 1987, and released version 1.0 to the comp.sources.misc newsgroup on December 18, 1987. The language expanded rapidly over the next few years. Perl 2, released in 1988, featured a better regular expression engine. Perl 3, released in 1989, added support for binary data.

Until 1991, the only documentation for Perl was a single (increasingly lengthy) man page. In 1991, Programming Perl (the Camel Book) was published, and became the de facto reference for the language. At the same time, the Perl version number was bumped to 4, not to mark a major change in the language, but to identify the version that was documented by the book.

Perl 4 went through a series of maintenance releases, culminating in Perl 4.036 in 1993. At that point, Larry Wall abandoned Perl 4 to begin work on Perl 5. Perl 4 remains at version 4.036 to this day.

Development of Perl 5 continued into 1994. The perl5-porters mailing list was established in May 1994 to coordinate work on porting Perl 5 to different platforms. It remains the primary forum for development, maintenance, and porting of Perl 5.

Perl 5 was released on October 17, 1994. It was a nearly complete rewrite of the interpreter, and added many new features to the language, including objects, references, packages, and modules. Importantly, modules provided a mechanism for extending the language without modifying the interpreter. This allowed the core interpreter to stabilize, even as it enabled ordinary Perl programmers to add new language features.

On October 26, 1995, the Comprehensive Perl Archive Network (the CPAN) was established. The CPAN is a collection of web sites that archive and distribute Perl sources, binary distributions, documentation, scripts, and modules. Originally, each CPAN site had to be accessed through its own URL. Today, the single URL http://www.cpan.org automatically redirects to one of several hundred CPAN mirror sites.

As of 2006, Perl 5 is still being actively maintained. Important features and some essential new language constructs have been added along the way, including Unicode support, threads, an improved support for object oriented programming and many other enhancements. The latest stable release is Perl 5.8.8.

Design

The design of Perl can be understood as a response to three broad trends in the computer industry: falling hardware costs, rising labor costs, and improvements in compiler technology. Many earlier computer languages, such as Fortran and C, were designed to make efficient use of expensive computer hardware. In contrast, Perl is designed to make efficient use of expensive computer programmers.

Perl has many features that ease the programmer's task at the expense of greater CPU and memory requirements. These include automatic memory management; dynamic typing; strings, lists, and hashes; regular expressions; introspection and an eval() function.

Larry Wall was trained as a linguist, and the design of Perl is very much informed by linguistic principles. Examples include Huffman coding (common constructions should be short), good end-weighting (the important information should come first), and a large collection of language primitives. Perl favors language constructs that are natural for humans to read and write, even where they complicate the Perl interpreter.

Perl syntax reflects the idea that "things that are different should look different". For example, scalars, arrays, and hashes have different leading sigils. Array indices and hash keys use different kinds of braces. Strings and regular expressions have different standard delimiters. This approach can be contrasted with languages like Lisp, where the same S-expression construct and basic syntax is used for many different purposes.

Perl has features that support a variety of programming paradigms, such as procedural, functional, and object-oriented. At the same time, Perl does not enforce any particular paradigm, or even require the programmer to choose among them.

There is a broad practical bent to both the Perl language and the community and culture that surround it. The preface to Programming Perl begins, "Perl is a language for getting your job done." One consequence of this is that Perl is not a tidy language. It includes features if people use them, tolerates exceptions to its rules, and employs heuristics to resolve syntactical ambiguities. Because of the forgiving nature of the compiler, bugs can be hard to find sometimes. Discussing the variant behaviour of built-in functions in list and scalar contexts, the perlfunc(1) manual page says "In general, they do what you want, unless you want consistency."

Perl has several mottos that convey aspects of its design and use. One is "There's more than one way to do it." (TMTOWTDI, usually pronounced 'Tim Toady'). Others are "Perl: the Swiss Army Chainsaw of Programming Languages" and "No unnecessary limits". A stated design goal of Perl is to make easy tasks easy and difficult tasks possible. Perl has also been called "The Duct Tape of the Internet".

Name

Perl was originally named "Pearl", after "the pearl of great price" of Matthew 13:46. Larry Wall wanted to give the language a short name with positive connotations; he claims that he looked at (and rejected) every three- and four-letter word in the dictionary. He also considered naming it after his wife Gloria. Wall discovered before the language's official release that there was already a programming language named PEARL and changed the spelling of the name.

The name is normally capitalized (Perl) when referring to the language and uncapitalized (perl) when referring to the interpreter program itself since Unix-like filesystems are case sensitive. Before the release of the first edition of Programming Perl it was common to refer to the language as perl; Randal L. Schwartz, however, forced the uppercase language name in the book to make the name stand out better when typeset. The case distinction was subsequently adopted by the community.

It is not appropriate to write "PERL", as it is not an acronym. The spelling of PERL in all caps is therefore used as a shibboleth for detecting community outsiders. However, several backronyms have been suggested, including the humorous Pathologically Eclectic Rubbish Lister. The more serious Practical Extraction and Report Language has prevailed in many of today's manuals, including the official Perl man page. It is also consistent with the old name "Pearl": Practical Extraction And Report Language.

The camel symbol

Perl is generally symbolized by a camel, which was a result of the picture chosen by publisher O'Reilly Media for the cover of Programming Perl, which consequently acquired the name The Camel Book. O'Reilly owns the symbol as a trademark, but claims to use their legal rights only to protect the "integrity and impact of that symbol" [5]. O'Reilly allows non-commercial use of the symbol, and provides Programming Republic of Perl logos (see above) and Powered by Perl buttons.

Future

Main article: Perl 6

At the 2000 Perl Conference, Jon Orwant made a case for a major new language initiative. This led to a decision to begin work on a redesign of the language, to be called Perl 6. Proposals for new language features were solicited from the Perl community at large, and over 300 RFCs were submitted.

Larry Wall spent the next few years digesting the RFCs and synthesizing them into a coherent framework for Perl 6. He has presented his design for Perl 6 in a series of documents called apocalypses, which are numbered to correspond to chapters in Programming Perl ("The Camel Book"). The current, unfinalized specification of Perl 6 is encapsulated in design documents called Synopses, which are numbered to correspond to Apocalypses.

Perl 6 is not intended to be backward compatible, though there will be a compatibility mode.

In 2001, it was decided that Perl 6 would run on a cross-language virtual machine called Parrot. This will mean that other languages targeting the Parrot will gain native access to CPAN and will allow some level of cross-language development.

In 2005 Audrey Tang created the pugs project, an implementation of Perl 6 in Haskell. This was and continues to act as a test platform for the Perl 6 language (separate from the development of the actual implementation) allowing the language designers to explore. The pugs project resulted in an active Perl/Haskell cross-language community centred around the Freenode #perl6 irc channel.

A number of features in the Perl 6 language now show similarities with Haskell, and Perl 6 has been embraced by the Haskell community as a potential scripting language.

As of 2006 Perl 6, Parrot, and pugs are under active development.

CPAN

Main article: CPAN

CPAN, the Comprehensive Perl Archive Network, is a collection of mirrored web sites that serve as a primary archive and distribution channel for Perl sources, distributions, documentation, scripts, and—especially—modules. It is commonly browsed with the search engine http://search.cpan.org/.

There are currently over 10,000 modules available on the CPAN, contributed by over 5,000 authors. Modules are available for a wide variety of tasks, including advanced mathematics, database connectivity, and networking. Essentially everything on the CPAN is freely available; much of the software is licensed under either the Artistic License, the GPL, or both. Anyone can upload software to the CPAN via PAUSE, the Perl Authors Upload Server.

Modules on the CPAN can be downloaded and installed by hand. However, it is common for modules to depend on other modules, and following module dependencies by hand can be tedious. Both the CPAN.pm module (included in the Perl distribution) and the improved CPANPLUS module offer command line installers that understand module dependencies; they can be configured to automatically download and install a module and, recursively, all modules that it requires.

Users without a C compiler are limited to pure Perl modules when downloading from the CPAN. ActiveState, a company that commercially supports Perl on Windows, provides packages of pre-compiled Perl modules, called PPMs, for use with their ActivePerl. Some pre-built modules are available as part of the MacPerl distribution for Mac Classic. Users on other operating systems often rely on binary packages from the operating system vendors. Due to licensing, availability of third party libraries, lack of portability, and many other issues, not all CPAN modules are available in pre-built packages.

Fun with Perl

As with C, obfuscated code competitions are a popular feature of Perl culture. The annual Obfuscated Perl contest makes an arch virtue of Perl's syntactic flexibility. The following program prints the text "Just another Perl / Unix hacker", using 32 concurrent processes coordinated by pipes. A complete explanation is available on the author's Web site. 

 @P=split//,".URRUU\c8R";@d=split//,"\nrekcah xinU / lreP rehtona tsuJ";sub p{
 @p{"r$p","u$p"}=(P,P);pipe"r$p","u$p";++$p;($q*=2)+=$f=!fork;map{$P=$P[$f^ord
 ($p{$_})&6];$p{$_}=/ ^$P/ix?$P:close$_}keys%p}p;p;p;p;p;map{$p{$_}=~/^[P.]/&&
 close$_}%p;wait until$?;map{/^r/&&<$_>}%p;$_=$d[$q];sleep rand(2)if/\S/;print

Similar to obfuscated code but with a different purpose, "Perl Poetry" is the practice of writing poems that can actually be compiled as legal (although generally non-sensical) Perl code. This hobby is more or less unique to Perl due to the large number of regular English words used in the language. New poems are regularly published in the Perl Monks site's Perl Poetry section.

Another popular pastime is "Perl Golf." As with the physical sport, the goal is to reduce the number of strokes that it takes to complete a particular objective, but here "strokes" refers to keystrokes rather than swings of a golf club. A task, such as "scan an input string and return the longest palindrome that it contains", is proposed and participants try to outdo each other by writing solutions that require fewer and fewer characters of Perl source code.

Another tradition among Perl hackers is writing JAPHs, which are short obfuscated programs that print out the phrase "Just another Perl hacker,". The "canonical" JAPH includes the comma at the end, although this is often omitted, and many variants on the theme have been created (example: [6], which prints "Just Another Perl Pirate!").

One interesting Perl module is Lingua::Romana::Perligata (or on CPAN). This module translates the source code of a script that uses it from Latin into Perl, allowing the programmer to write executable programs in Latin.

The Perl community has set aside the "Acme" namespace for modules that are fun or experimental in nature. Some of the Acme modules are deliberately implemented in amusing ways. Some examples:

See also

Notes


Books

Perl

(Also see Books under External links, below.)

Perl man pages

The Perl man pages are included in the Perl source distribution. They are available on the web from http://perldoc.perl.org/. Some good starting points are:

Web pages

Major Resources

Development

Books

(Also see Books under References, above.)

Support

Distributions

Miscellaneous

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