Social momentum is also a thing; which mostly expands on point 5, but also points 1 and 2.
IF you are going to find Perl installed on some system, it's more likely to be Perl 5 than anything newer.
You are very likely to find SOME version of Python, or Python compatible thing on most Linux distributions. Anything really old you're safe targeting Python 2.6 compat, maybe 2.7 compat. Anything relatively recent you will probably find a 'decent' version of Python 3.
IF you are going to find Perl installed on some system, it's more likely to be Perl 5 than anything newer.
You are very likely to find SOME version of Python, or Python compatible thing on most Linux distributions. Anything really old you're safe targeting Python 2.6 compat, maybe 2.7 compat. Anything relatively recent you will probably find a 'decent' version of Python 3.