If your main concern is deploying crossplatform solutions then I wouldn't look too hard at Gtk because if it's dependancy on Gnome. Tk on the other hand is certainly a good candidate for behaving on multiple platforms. Not only that, it is well established, well supported, and there's a ton of code out there (just do a search for Tk on CPAN to see what I mean).

If you do decide to go the Tk route, picking up a copy of Mastering Perl/Tk would certainly be worth your while.