You can begin exploring from either http://www.perl.com/CPAN/ or any of the mirrors listed at http://www.cpan.org/SITES.html and http://mirror.cpan.org/.
Note that CPAN is also the name of a Perl module, CPAN.pm, which is used to download and install Perl software from the CPAN archive. This FAQ covers only a little about the CPAN module and you may find the documentation for it by using perldoc CPAN via the command line or on the web at http://theoryx5.uwinnipeg.ca/CPAN/perl/CPAN.html.
