Personally if you're looking to learn another language I wouldn't bother with Perl. It's close enough to what you already know that you probably wont learn much [1]. You'll get more milage out of trying something like Smalltalk (Squeak is free and a decent environment) if you want to really learn OO inside out, Haskell if you want to learn functional (this will bend your mind for a while at first, don't give up!), Erlang could be interesting but very unlike most other things.
[1] You can probably learn things in Perl that you couldn't in PHP but if you pick a more pure language the lessons will stick out more. Perl also has a lot of extremely bad practices by default.
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Personally if you're looking to learn another language I wouldn't bother with Perl. It's close enough to what you already know that you probably wont learn much [1]. You'll get more milage out of trying something like Smalltalk (Squeak is free and a decent environment) if you want to really learn OO inside out, Haskell if you want to learn functional (this will bend your mind for a while at first, don't give up!), Erlang could be interesting but very unlike most other things.
[1] You can probably learn things in Perl that you couldn't in PHP but if you pick a more pure language the lessons will stick out more. Perl also has a lot of extremely bad practices by default.