What is that supposed to mean? How do you "mathematically" compare programming languages (and yes I do know about denotational semantics)?

All languages are same in a mathematical sense, that what you can do in one turing complete language you can do in another. Albeit with a lot of pain and kungfu

when you abstract away inessential syntactic differences...

Firstly, those are not 'inessential' syntactic differences. Because the very purpose Perl was invented was to solve some very essential problems. Problems both at both programmer level and the problems programmers try to solve. This continues to be the case till today.

So if you look at the whole range of languages from assembler to C and C++, Lisp, Haskell, Prolog and so on the ecological niche of Perl, Python and Ruby is exactly the same

You keep talking about 'small syntactical differences'. Those are not small. I solved a very pressing P1 issue on production today morning with Perl in something like 25 minutes. I can't imagine doing that in C++. Its just not possible!

Lastly, I think you are stuck in this principle called Occam's razor- Which basically says "All things being same, the simplest thing turns out to be the best". The only problem is there are more than one condition to be considered while deciding if two things are the same