I've tried Tk, Gtk, and Wx. Of them, I find that Wx has the weakest documentation by a long shot, although, to be fair I haven't actually looked at it since 2003 or so.

I like Gtk the best iff (if and only if) there's an OS package for it. It's a complete nightmare to install via CPAN and I don't say that about very many CPAN modules — in fact, just that one. I've definitely had it install flawlessly from CPAN, but that is the exception.