At least in the area of web frameworks perl could profit from more choice. And even if there were only one python and one ruby library for lets say XML-manipulation, it would be two more for perl.

If the only reason for perl to exist were CPAN it should die. If not, there will be people changing from python or ruby to perl and not only in the other direction.

One big advantage of open source is lots of users that find bugs, locate bugs and even fix bugs. I would not mind pythonistas finding or fixing my bugs.