You could use Devel::Symdump to dump everything in the main:: namespace. Several other of the Devel:: modules might also be of interest to you.

As for understanding symbol tables, http://www252.pair.com/comdog/mastering_perl/Chapters/08.symbol_tables.html, perlguts and http://blob.perl.org/tpc/1998/Perl_Language_and_Modules/Perl%20Illustrated/ might help.