Well, Tk has it's good and bad sided. The big, big BIG bonus is: It works on every operating system i ever tested it without any major obstacles. But of course, it looks old fashioned (because it is) and it's quite clumsy to program a more complex interface.

If you want to design UIs with Tk, you can try ZooZ. It's a few years since i used it last, but it worked back then.

As for other user interfaces: There are modules for Gtk2 and Gtk3. I wrote a simple example 944371 that uses GTK3 plus WebKit to make a simple one-purpose webbrowser. (Most of the article actually deals with fixing MakeMaker to work correctly in 64bit Linux).

In theory there are also some Qt bindings, but as far as i know, they haven't been updated for quite a while.