I'm usually not big on advocating learning various languages. Overall, I believe that good programmers will do well with any language you throw at them. If you learn good programming skills, you will never have a problem having a job.

However, since you asked, I'd have to agree with the C advocates above. First of all, C can help you with your Perl allowing to create additional modules with XS. Second, a solid understanding of Perl will give you the more than enough background to understand C++, Java, or C#. Your advantage will be that you will understand memory allocation, so you'll right better, tighter code in those languages than various "experts".