Well here are some things to consider:

1. Qt is what is used by KDE, but the Perl interface to it is practically useless, due to lack of interest or whatever. The last release was in 2003. So if you want to program in Perl, forget Qt. Qt using C is very good, especially for KDE.

2. Gtk2 is very good, and the development is very active, just look at it's maillist for c-gtk2 and perl-gtk2. It has good tutorials, and alot of modern widgets. It can be a bit rough to learn as a first language though. It's neat feature is that if you can do it in Perl, you can convert it to C, since the libs are the same. Portability can be an issue with Gtk2. It is still a bit daunting to get the Gtk2 system installed properly on Windows. (Don't know about Mac's). Gtk2 apps have themes, and will take the same theme as defined in ~.gtk2rc . They will look like Mozilla or Firefox on a Linux system.

3. Tk is very well supported and the best cross-platform compatibility out there. It comes installed with ActiveStatePerl, so Window users get it automatically. Tk has many supporters and snippets out there, as well as books written. "Mastering Perl/Tk" is the bible. You can find help to do almost anything with Tk, since there are so many existing users... this is not so true with Gtk2, where even the experts get stymied sometimes as to why something won't work. The biggest complaint you hear about is Tk apps look drab, but you can make the Canvas or Zinc widgets look great.....depending on effort put in. As a matter of fact, The Tk::Canvas and Zinc widgets are the number1 reason to recommend Tk over the others....they are still king of the hill. The downside to Tk is the lack of an active group of developers for the module itself. The module originator, Nick Ing Simmons, died recently, and there has been no one leader (except for Slaven Reszic) to take the reign. This is worrisome.

So if I was starting new, and willing to put some time in learning, I would go with Gtk2..... there is a whole team of developers very actively working, and that portends well for the future.