I may blog about this next year, again[^1], as I'm working on a project that sort of covers it - not in a way that will answer the question but more observational.

Anyway, I feel Perl's popularity was hugely exaggerated in the mid to late 90s and early 00s. The alternatives were either not there in terms of language and toolchain features, ease of use, "whipuptitude" or whatever, or library support (CPAN was a killer app), or they were too old school or enterprisey. Sysadmins were using it everywhere so it got into all sorts of systems that other languages couldn't without much more faff.

Its back compatibility meant it stayed in those places for a long time. It's still in a lot of those places.

The fall in popularity the last decade or two was more of a regression to the mean, or perhaps below the mean. Many other languages have come along, which have contributed even more to the fall in share.

Yes, yes, Raku (né Perl 6) but I'd argue that also contributed to a lot of really good stuff on CPAN. The Perl 5 core did get neglected for a number of years, as @autarch says, which may have been a factor.

[^1] previously: https://leejo.github.io/2017/12/17/tpc_and_the_end_of_langua...