A Perl module is a discrete component of software for the Perl programming language. A module is distinguished by a unique namespace, e.g. "CGI" or "Net::FTP" or "XML::Parser". By conventions, there is typically one module per file with a .pm extension. A collection of one or more modules, with accompanying documentation and build scripts, compose a package. The Perl community has a sizable library of packages available for search and download via CPAN.
It is common for Perl modules to have embedded documentation in Perl's [[Plain Old Documentation|POD]] format. Many modules favor an object-oriented style, but many are procedural instead, especially old modules.
Below is an example of a very simple object-oriented Perl module and a short program which makes use of the module. It is implemented in a dialect of Perl5 which is compatible with Perl 5.6.0 and higher.
helloworld.pl
#!/usr/bin/perl -w use Hello::World; my $hello = Hello::World->new(); $hello->print();
Hello/World.pm
# The 'package' command gives the name of the module or class.
package Hello::World;
use strict;
use warnings;
# By convention, a module's version number is stored in
# $ModuleName::VERSION; certain forms of the "use" built-in depend
# on this variable being defined.
our $VERSION = "1.0";
# Lines starting with an equal sign indicate embedded POD
# documentation. POD sections end with an =cut directive, and can
# be intermixed almost freely with normal code.
=head1 NAME
Hello::World - An encapsulation of a commonly output message
=head1 SYNOPSIS
use Hello::World;
my $hw = new Hello::World();
$hw->print();
=head1 DESCRIPTION
This is an object-oriented library which can print the famous "H.W."
message.
=head1 METHODS
=over
=item new
Instantiates an object which holds a greeting message.
=cut
sub new {
my $pkg = shift;
my $self = bless({
message => "Hello, world!",
}, $pkg);
return $self;
}
=item to_string
Returns the greeting as a string
=cut
sub to_string {
my $self = shift;
return $self->{message};
}
=item print
Outputs the greeting to STDOUT
=cut
sub print {
my $self = shift;
print $self->toString(),"\n";
}
=back
=head1 AUTHOR
Joe Hacker <joe@joehacker.org>
=cut
# The lone "1;" at the end of the file indicates that the module
# has been successfully initialized. Some modules contain code
# that runs as soon as the module is compiled, before the program
# that included it is finished compiling; they can arrange to return
# a non-true value if something goes wrong.
1;