Well, there is the perl Gtk bindings, but I believe it is incomplete and has little to no documentation. So it's not really a choice. There is the Gnome bindings, but that is linux only. I wonder if Qt has perl bindings... But I know nothing about Qt and even less about C++. :-)

My real suggestion is wxWindows. It's cross platform & uses native widgets. However someone told me last night that it was evil. Unfortunately, his subway stop was long before mine so I didn't get his full argument. It was something about it being very hard to subclass widgets to do custom stuff. I've heard good things about wxWindows from other people who are using it in real code, so YMMV.