Although I haven't done much GUI work in Perl yet, I've been very pleased with WxWidgets so far. It's not as powerful as GTK+, or as pretty always (even though it uses GTK+ as a backend on unix, there are sometmes holes where real GTK+ apps look better) but it was easy to learn and WxGlade can output perl code for quick prototyping.

The other big advantage of Wx is that it uses native toolkit widgets instead of drawing its own. That way you can ship programs to people on different platforms and it will look like most of their other applications, as opposed to things like Java Swing which look the same on all platforms but nothing like other programs on those platforms.

Tk was popular in the past probably because it was easy to use, but now that it's so ugly and clunky compared to modern toolkits, the extra effort to learn Wx or GTK+ is probably more worth it now.