"PM" is used here for PerlMonks, Private Message, Perl Module, Perl Monger, and Evening (at least). "Send me a PM on PM about the PM PM PM tutorial." (Update: I just got a PM PM that used "PM" to, I assume, mean "project manager")
Most of these are common net acronyms that you can find many places (if a few of the best aren't listed in the Monk Help, then we should add such). The ones that aren't should get included in the Monk Help on acronyms (Guide to PerlMonks Acronyms and Abbreviations -- found quickly via Super Search).