but it's also ancient and (from what I can tell) poorly-documented

I beg to differ.

Tk's recent update is Dec 2007, which isn't "old". The ToolKit has been around for a while (since about 91), but so has wx (92), and I prefer something that is being developed for awhile rather than a new toolkit that might still have some wrinkles and dark corners.

Tk is also as well documented (as any other Perl module), and has a whole book dedicated to it (Mastering Perl/Tk, ISBN 1565927168), which isn't something other Perl GUI ports/bindings has. It's also very Perlish in its API and very comfortable to work with in a Perl environment.