From what I've seen, and used it for, Tk is very useful for small/medium-sized apps, because in some ways it's a slightly limited module. Having said that, though, it's certainly possible to do some very complex things with it, if you're willing to think through the problem carefully.

The other advantage of Tk is that it's platform-independant - it'll work just as well on Windows as it will on Unix, which can be a huge benefit if you need that cross-platform capability. The downside to this is that it uses the Tk widget set, which means that it looks similar, but not quite the same as, the platforms in question.

Win32:GUI I'm not so familiar with - but from what I've read it's a similar tool to Tk, but Windows-native. It looks fairly powerful, and, as with Tk, the documentation looks very extensive.

There're also options like wxPerl, which is a very usable Perl wrap-around to the wxWindows toolkit.

My personal choice on a day-to-day basis is Tk - it's so far allowed me to do everything I've needed to, and I find that the cross-platform functionality is invaluable in my workplace (we use both NT and Unix boxes). It's also pretty quick and easy to knock a working GUI together, and there're some very good tutorials and books out there about the module.

Having said that, I'm a firm believer in the right tool for the right job - so what you're actually going to use it for is much more important, in my opinion ...

As runrig says, though, take a look at the FAQ - there's a lot more information on there.

Just some quick thoughts ..
-- Foxcub