Interesting. I know python and ruby. Now I'm wondering if it's worth my time to learn perl.
Python and Ruby are not that different, and neither is that different from Perl.
Disclaimer: I quite like JavaScript.
If you want to learn about prototype-based OO systems, fine, but there's a reason that only JavaScript (and Self) use prototype-based OO :P
C should be learned so that you are not tempted to ever use it for anything important.
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.
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.
That was a design decision, not influence from natural languages? Fun.