IUP is great (it uses Gtk on GNU/Linux and native WinAPI calls on Windows; it also makes programming GUIs in C almost enjoyable despite the fact that you are writing a GUI in C), but I'm not sure about the quality of the Perl bindings. Last time I wanted to write a GUI program, I ended up learning a new language :(

Prima is also an interesting toolkit; relatively easy to install and well maintained (getting a few commits per month since early 2000-s till now), but it looks the same on every platform.

Of course, it's really hard to topple the popularity of Tk. Despite looking dated overall and it belonging to a different language (Tcl), the toolkit is still used by many other languages, including Perl, R, and Python. Chances are that Tk is already installed on the target machine, unlike IUP and Prima.