If you know python and ruby, learn something that I will actually improve/change your programming: Scheme, ML, Prolog, Smalltalk, Common Lisp, Haskell, Mozart/Oz, or Forth.
Python and Ruby are not that different, and neither is that different from Perl.
I'll differ from berntb - you should learn Perl exactly because it's fairly similar to those languages you already know, and the effort required to do so will be minimal.
You may not end up using it all that frequently, but an hour or two seeing how another programming language does something is usually interesting perspective, imo.
IMHO, the best part with Perl is the community (CPAN is a result of that) -- and the worst part is the relentless trolling on web sites like this...
Since Ruby/Python/Perl are really similar, I'd recommend that you learn something completely different instead.
Disclaimer: I like Perl. What makes Perl fun for me is to a large part that the linguistics influence results in Perl breaking most "normal" laws for programming language design, but still works. I go to work smiling.
I don't think it's a lingusitics thing, rather, Perl was not designed to be limited like other popular languages were. Larry never sat down and said, "I am the only person that is possibly smart enough to understand this concept, so I am not going to add it to the language". (Java and PHP's designers did. Look what that got them...)
Python and Ruby are not that different, and neither is that different from Perl.