php would be useful if/when you're doign anything web-based (php & perl work well in tandem)...knowing C/C++ is always a good thing for a perl programmer (perl, among others, is written in C, so any perl problems are inherently C problems). so php and mabye C (depending on your future projects) would be my suggestions for you

addendum: though you can do just about anything in perl, sometimes perl isnt the best way. for instance, if you're aiming for fast, small, standalone executables, C would be the way to go. if you're looking for cross-platform with minimal porting, Java. it's not always "you can do that in perl", sometimes its "you can do that more efficiently in ******* language".