Perl's decline was cultural

Python is mentioned and I think the key reason it's continued to grow while Perl declined, is a vastly more welcoming culture.

Python says you know nothing, but want to automate a small task. The community will help you. More so than any other language.

Then again, Python 2 and Python 3 are two different languages.

Very few projects are willing to have such a massive migration.

"Willing" is an interesting word choice. There was quite a bit of resistance in the Python world despite the clear benefits. (2.x really could not be fixed, because the semantics were fundamentally broken in many places.)

It's open source.

Any one ( and I'm sure a few have tried) can fork 2.x and keep using it.

3.x is remarkably easy , you can probably onboard a non programer to Python in a month.