I have no experience with GTK, but TK is quite multi-platform (it works on X and it works on Windows - in ActiveState package and with a patch on Cygwin). I cannot compare it to other GUI tool kits, but for me it was a bit buggy - I don't mean anything critical but some inconsequences in appearence.

If all you need is to display a stack of rectangles you might use just the GD library to generate a picture (a png or a gif).

The difference between Perl and C/C++ is a bit broad subject and I don't feel competent to talk about it, but for sure you can write applications in Perl, you even can write graphical applications in Perl if only you use a good library for the low level data munging.