|laziness, impatience, and hubris|
For "general work around my system" Perl, Python or Ruby are equally valid ways to go.
But if you have to work a lot on systems that are not your own an investment in Perl may be better as it is ususally available on all Unix-systems.
Also I'd say to become really proficient in Perl takes more time but the upside would be that as a Perl-hacker you ususally can deal with Python-code (the converse is usually much harder).
For non-productive uses of programming languages (obfuscation, golf, poetry) Perl does not really have a contender but that may or may not be your cup of tea.
There are however many interesting developments in the Python-world (e.g. PyPy) and I think if I had only limited time and only wanted a tool for my own system I'd go for Python.
But if you can afford a somewhat steeper learning-courve Perl might be a (slightly) better investment.
But I guess it very much comes down to one's taste.
Some people just love Python's "there is one right way to do it" approach, some prefer Perl's TIMTOWTDI.
There is no "better" there, just different philosophies.
Try one, if you like it stick with it, if not change to something else.