Perl/Tk. Possibly the most widely known GUI for Perl
Even better, read the source code for Perl itself (although that may be really hard if you don't know C since Perl is partially written in C).
Muhahaha. You're right that Perl is only partially written in C, it's mostly written in C-preprocessor macros. But reading the perl source code to make you a better Perl programmer is like saying that reading the DNA sequence of roses makes you a better gardener.
By reading the source code, you are looking at all of the features of the module, and how far it can really go
Really? How does that work? Most modules could "go" much further than they are going, but how do you determine how far it could "go"?
Finally, keep up with Perl. I know it may be kind of hard, but it can help you use all that Perl has to offer. Also, make sure your Perl version is up to date (or at least almost up to date) so that everything is compatible.
I doubt so. Tell me, which language features did 5.12 and 5.14 introduce that makes one a much better programmer? Heck, one could even ask that about 5.8 and 5.10. Surely you aren't thinking that using (*MARK)
inside a regexp, or smart matching makes you a better programmer?