Here's my little list:

• Any of the Lingua::* Modules
• Parse::RecDescent - Yes I still haven't really used it.
• Net::AIM / Net::Gnutella / Net::ICQ
• Audio::MPEG and related modules.
• Image::Magick - I Really want to learn how to use this better as well.
• NetPacket
• Chemistry::Elements
• More of the Crypt::* modules.
• WWW::Robot - To see how superior it is to my messy hack.

Update: I really should do some more work with TK as well :).