The Perl 5 module BackPAN::Index provides an index of BackPAN and some handy methods. The Comprehensive Perl Archive Network (CPAN) is a very useful collection of Perl code. However, in order to keep CPAN relatively small, authors of modules can delete older versions of modules to only let CPAN have the latest version of a module. BackPAN is where these deleted modules are backed up. It's more like a full CPAN mirror, only without the deletions.