Installing a new version of Perl doesn't break your old code! This is not true for Ruby (I'm not sure about Python as I no longer use it much), which is a huge argument in favour of using Perl over Ruby.

Having said that, it's certainly possible to use all of these languages, and once you are good at one, it should take little extra effort to learn the others. All of them have their strong points and you'll probably regularly see code in each of them, so just learn them all.