short answer -- look at use Data::Dumper; print Dumper \%INC; or Module::Depends or Module::ScanDeps (which is what PAR uses--can always just use pp and see what it bundles up).
long answer -- browse these nodes:

• What modules are we actually using?
• Which modules does somescript.pl use?
• finding all modules used from a script

And on a related topic:

• existing PERL modules?
• Installed Module
• Listing all installed modules
• Search @INC And Display Modules
• Detecting if a script uses non-standard module