Running: grep -h "^use" *.pl | grep -v -E "constant|strict|warnings|vars" | sort | uniq on the last couple of projects I've worked on gives me the following list of modules:

Apache::Session::MySQL, CGI, CGI::Carp, Class::DBI, Data::Dump, Data::FormValidator, Digest::MD5, Email::Valid, HTML::Entities, HTML::Parser, HTML::TagFilter, HTML::TreeBuilder, Mail::Sendmail, MIME::Lite, Storable, Template, Text::ParseWords, Time::Piece::MySQL, URI::Escape, YAML

I'd consider them to be the most useful modules I regularly use.