If you are just starting to learn, I would stick with Tk. It has more documentation, users, and snippets floating around than any other alternative. Tk will make it easy to learn the first steps needed in understanding GUI's. Gtk2 has quite a steeper learning curve, and you may get discouraged trying to learn with it. As far as OpenGL and Perl goes, SDL is the only perl module that has it working effectively.

None of the toolkits has a really easy to use rapid development tool, like you might want (i.e. like Visual Basic). The closest to that would be Glade and Gtk2. My question is WHY?. Why do you want an alternative to Tk? Did someone say it wasn't worth learning? If you want to get right into the nitty gritty, and be able to use the widest possible variety of robust tools, learn the C version of Gtk2. C is where everything is at. When something at the C level becomes very popular, and in demand, then people port it to Perl (and Python/Ruby) ports of it, to make it more accesible.