You can pretty much do anything you want in either language. Just keep in mind that perl is a scripting (interpreted) language, whereas C++ is a compiled language. If you're looking to write security tools and whatnot, you can use either one. I personally use C and C++. I've seen portscanners, web-vulnerability testers, servers, etc, written in both perl and C++, so it's really a matter of preference more than anything. Besides, either one will get you a job if you're proficient enough.

-noix-