GUI toolkits come and GUI toolkits go, but Tk is still standing strong because of it's ease, wide user base to help answer questions, and documentation. The documentation available for Tk is better than for any other toolkit out there. Try Tk Tutorial and uwinnipeg Tk tutorial, and if you really want to become expert, see the Tk bible at Mastering Perl/Tk. You will not find better documentation for any other GUI toolkit.

There are other GUI toolkits out there, namely gtk2-perl and WxPerl. They are state of the art, and evolving so fast it's hard to keep track. Needless to say, they are harder to use and understand the code.

Finally, the Tk userbase is so big and experienced, that you will have little trouble getting help with problems as you learn.

So yes, use Tk;. At the very least, it will help you quickly learn and get a GUI up and running. Then, if you want to move to the more advanced toolkits, you will already have a headstart on what is going on with things like the eventloop and packing. I pretty much can guarantee that once you learn Tk, you will find yourself using it, even if you know the other GUI toolkits, because of it's simplicity and ease of use.