Wx is relatively new, Tk has been around a long time and is well supported and stable.
The lack of layout design tools for Tk does make it somewhat harder to get a really good looking interface. I have just finished a new application using Perl/Tk. I used Tk for several reasons.
Now Wx is nice! I looked at it, and I will look at it again. I will also look at Prima - yet another GUI toolkit for Perl. As always in this wonderful Perl culture - TIMTOWTDI. And lastly, for some of us, Tk is what is available to us. In many sites where Iwork I am restricted to either CPAN on Linux or Activestate PPM's for Windows. There are no PPM packages for the Wx environment.