Actually, I can think of a lot of things that I know only from participation in forums like this. These tend to be things that someone else found out by banging on it. Some of them actually get officially documented somewhere but I think many of them still aren't.

Another good source of undocumented secrets is the source code (the source code for Perl, for modules, for the Microsoft C RTL).

The problem with %SIG handlers has been discussed a few times in the comp.lang.perl.* newsgroups (I don't read comp.lang.perl.misc anymore so it was probably most recently in comp.lang.perl.moderated).

Another great forum is p5p. I subscribe to the weekly summary but we've had another long gap in summaries so I'll have to resort to schlepping the web archive for interesting threads if I want to stay "on top".