You are already familiar with PHP so you can easily learn Perl or ruby because you already know how to define condition or any other fundamental concept in PHP so in Perl or ruby you just need to understand coding syntax rather than whole fundamental concept.
It's always good to learn new languages, it keeps you on your toes. Ruby on rails is quite popular so that is a fine one to go with. Eventually it will be no big deal, I've picked up languages in a day because I know so many others. PERL is really only used in legacy systems but it's still worth knowing.