A vast collection of Free Perl software and documentation. If you're a Perl programmer, you're the kid and CPAN is the candy store. See the CPAN main page for more info and lots of sweets. The quickest way to use CPAN to get modules is perl -MCPAN -eshell.
However, many of the more popular modules are available as packages for DebianLinux, RedHat and various other LinuxDistributions, so you're better off using the PackageManagement if you can. The naming schemes for the packages vary. DebianLinux lowercase the distribution's name, sometimes arbitrarily add dashes to it, and drop it in libdist-perl, while RedHat simply use the distribution's name. So the DBD::Pg module from the DBD-Pg distribution is contained in the packaged called libdbd-pg-perl in DebianLinux and DBD-Pg in RedHat.