Installing it is a breeze, learning it is easy, and the widget set is very nice and mature in terms of reliability and use. The canvas widget deals with graphics easily, though if you're looking to make a first-person-shooter it won't be sufficient. I typically just create the graphics with GD or something, then attach them to a canvas, though there are native functions for drawing directly on the canvas.

While you do have to have the Tk module installed on any machine that will be running your scripts, you can also purchase PerlApp from ActiveState, which will allow you to turn your Perl scripts into binaries on Solaris, Linux, and Windows. I've used it before to build rather robust Perl/Tk applications, replete with complex compound widgets etc., and it works like a charm. The Windows users typically never realize they are running something written in Perl and the code gets written in half the time.

Hope this helps :-)