Other than a few that you mentioned, here are the ones that I have been putting off using:

1. Parse::RecDescent - I tend to do most of my parsing with split and regex (and specific modules, such as HTML::Parser, etc), and I can usually get what I need done without much problem. However, I often get the feeling that familiarity with Parse::RecDescent could save me tons of work.
2. Win32::* - I work in Windows a lot of the time, and it would be nice to know what I could actually use it for (Windows, I mean) ;)
3. POE - For all of the attention it gets, it HAS to be good, right? :) But seriously, reworking code to use this module might be very handy if I find myself forking off waaaay to many processes.
4. Inline - Because its so damn neat.