Like many programming languages, Perl has mechanisms to use external libraries of code, allowing a single file to contain common routines used by several programs. Perl calls these modules. Perl modules are typically installed in one of several directories whose paths are placed in the Perl interpreter when it is first compiled; on Unix-like operating systems, common paths include /usr/lib/perl5, /usr/local/lib/perl5, and several of their subdirectories.
Perl comes with a small set of core modules. Some of these perform bootstrapping tasks, such as , which is used for building and installing other extension modules; others, like CGI.pm, are merely commonly-used. The authors of Perl don't expect this limited group to meet every need, however.
The CPAN's main purpose is to help programmers easily locate modules and programs not included in the Perl standard distribution. It is also used to distribute new versions of Perl, as well as related projects, such as Parrot.
The CPAN is an important resource for the professional Perl programmer. With over 12,000 modules (containing 20,000,000 lines of code) the CPAN can save programmers weeks of time, and large Perl programs often make use of dozens of modules. Some of them, such as the DBI family of modules used for interfacing with SQL databases, are nearly irreplaceable in their area of functionality; others, such as the module, are simply handy resources containing a few common functions.
In 2003 distributions started to include metadata files, called META.yml, indicating the distribution's name, version, dependencies, and other useful information; however, not all distributions contain metadata. When metadata is not present in a distribution, the PAUSE's software will usually try to analyze the code in the distribution to look for the same information; this is not necessarily very reliable. (See the Uploading Distributions with PAUSE section for more.)
With thousands of distributions, CPAN needs to be structured to be useful. Distributions on the CPAN are divided into 24 broad chapters based on their purpose, such as Internationalization and Locale; Archiving, Compression, And Conversion; and Mail and Usenet News. Distributions can also be browsed by author. Finally, the natural hierarchy of Perl module names (such as "Apache::DBI" or "Lingua::EN::Inflect") can sometimes be used to browse modules in the CPAN.
CPAN module distributions usually have names in the form of CGI-Application-3.1 (where the :: used in the module's name has been replaced with a dash, and the version number has been appended to the name), but this is only a convention; many prominent distributions break the convention, especially those that contain multiple modules. Security restrictions prevent a distribution from ever being replaced, so virtually all distribution names do include a version number.
The heart of CPAN is its worldwide network of more than 260 mirrors in more than 60 countries. CPAN's master site, ftp.funet.fi, has over 149 direct public mirrors. Each site contains up to the full 3.9 gigabytes of data, or a subset of it if the mirror's maintainer wishes to selectively choose.
Most mirrors update themselves hourly, daily or bidaily from the CPAN master site. Some sites are major FTP servers which mirror lots of other software, but others are simply servers owned by companies that use Perl heavily. There are at least two mirrors on every continent except Antarctica.
For more information on CPAN mirrors, see mirrors.cpan.org
Several search engines have been written to help Perl programmers sort through the CPAN. The most popular and official is search.cpan.org, which includes textual search, a browsable index of modules, and extracted copies of all distributions currently on the CPAN. The official cpan search module often provides a suboptimal experience, other more modern search engines that have arisen are:
CPAN Testers are a group of volunteers, who will download and test distributions as they are uploaded to CPAN. This enables the authors to have their modules tested on many platforms and environments that they would otherwise not have access to, thus helping to promote portability, as well as a degree of quality. Smoke testers send reports, which can are then collated and used for a variety of presentation websites, including the main reports site, statistics and dependencies.
A family of other loosely integrated support websites have been created as the CPAN has grown in size and scale. These are created and managed by individual Perl developers, and provide data feeds to each other in various ad-hoc ways.
There is also a Perl core module named CPAN; it's usually differentiated from the repository itself by calling it CPAN.pm. CPAN.pm is mainly an interactive shell which can be used to search for, download, and install distributions. An interactive shell called cpan is also provided in the Perl core, and is the usual way of running CPAN.pm. After a short configuration process and mirror selection, it uses tools available on the user's computer to automatically download, unpack, compile, test, and install modules. It is also capable of updating itself.
Recently, an effort to replace CPAN.pm with something cleaner and more modern has resulted in the CPANPLUS or CPAN++ set of modules. CPANPLUS more cleanly separates the back-end work of downloading, compiling, and installing modules from the interactive shell used to issue commands. It also supports several advanced features, such as cryptographic signature checking and test result reporting. Finally, CPANPLUS can uninstall a distribution. CPANPLUS is expected to replace CPAN.pm in the core distribution in Perl 5.10.
Both modules can check a distribution's dependencies and can be set to recursively install any prerequisites, either automatically or with individual user approval. Both support FTP and HTTP and can work through firewalls and proxies.
Authors can upload new distributions to the CPAN through the Perl Authors Upload Server ( PAUSE). To do so, they must request for a PAUSE account A list of requirements for registration can be found at the PAUSE faq
Registration is not immediate, and typically takes a week.
Once registered, the new PAUSE account has a directory in the CPAN under authors/id/(first letter)/(first two letters)/(author ID). They may use a web interface at pause.perl.org, or the PAUSE ftp server to upload files to their directory and delete them. PAUSE will warn an administrator if a user uploads a module that already exists, unless they are listed as a co-maintainer. This can be specified through PAUSE's web interface.
Experienced Perl programmers often comment that half of Perl's power is in the CPAN. Though the TeX typesetting language has an equivalent, the CTAN (and in fact the CPAN's name is based on the CTAN), few languages have an exhaustive central repository for libraries. The PHP language has PECL (PHP Extension Community Library) and PEAR (PHP Extension and Application Repository), and Python has a PyPI (Python Package Index) repository, but neither is as large nor as active as the CPAN. Other major languages, such as Java and C++, do not have anything similar to the CPAN (though for Java there is central Maven , and C/C++ has the Boost C++ Libraries).
The CPAN has grown so large and comprehensive over the years that many people learning Perl seem to elevate it to a sort of mythical status, and express surprise when they begin to encounter topics for which a CPAN module doesn't exist already.
The CPAN's influence on Perl's eclectic culture should not be underestimated either. As a hive of activity in the Perl world, the CPAN both shapes and is shaped by Perl culture. Its "self-appointed master librarian", Jarkko Hietaniemi, often takes part in the April Fools Day jokes so popular on the Internet; on 1 April 2002 the site was temporarily named to CJAN, where the "J" stood for "Java". In 2003, the www.cpan.org domain name was redirected to Matt's Script Archive, a site infamous in the Perl community for having badly-written code.
Beyond April Fools', however, some of the distributions on the CPAN are jokes in themselves. The Acme:: hierarchy is reserved for joke modules; for instance, Acme::Don't adds a
don't function that doesn't run the code given to it (to complement the
do built-in, which does). Even outside the Acme:: hierarchy, some modules are still written largely for amusement; one example is Lingua::Romana::Perligata, which can be used to write Perl programs in a subset of Latin.
Over the years, the CPAN has had a range of unusual non-Perl things uploaded to it.
The following are just a few examples.