Some people actually find Perl fits the way they think better than Python does. They may find the culture a better fit, or they're better able to find existing solutions, or they simply prefer the concept of "There's more than one way to do it" over "There should be one – and preferably only one – obvious way to do it." Finally some are better paid to write Perl than they are to write Python.

The reality is any/some/all of the reasons above apply to any combination languages in your question. Might as well ask "Why are programmers still using COBOL rather than Erlang?" Because.