Well I guess most would say you use a language to solve a problem, they're a tool. So if one language would make it easier to solve a problem than another, use the easier of the two.
As you dont mention a specific problem you intent to use Python or Perl to solve, think up an interesting project/problem you want to solve, then find which is the better language to solve it in. Then try solving it in the other language once you've developed your solution
And now after all those buzzwords: PYTHON!
Mixed with wxPython it's so easy to make fancy gui apps, and the interactive nature of Python makes coding and testing faster (no need to compile, thought there is a compiled form to speed things up)