Maybe the debian guys did something goofy with the modules. It wouldn't be the first time. That or maybe its just a bug in ModPerl. Even if some CPAN module out there has 'use warnings;' in it that shouldn't have any affect on any code outside of its own package should it?
I think that Moose and Modern::Perl do something fancy to globally turn on both strict and warnings, but aside from that I have no idea off hand.
Adam