If you need a "second" language in terms of usage, you may be glad to find out that Perl can fulfill all your needs. Especially in a web envorinment - Perl is terrific there.

You may not find Perl sufficient in some rare cases. For instance, if you really need super-efficiency. Then, knowing C or C++ is a good idea. But this is very rare, especially for web needs.

The other monks gave you very good advices about languages to "open your head". Functional languages are terrific, even if you won't use them - they contain a wealth of useful ideas, most of which are implemented in Perl (!!). Learning assembly can help you understand computer architecture, though I wouldn't advice it, unless you need to code for performance.