There are many languages still in use today that have all kinds of warts and ugliness, but they remain in use because they still have momentum and lots of legacy things built in them. So being ugly or old isn’t enough of a factor for people to abandon something in droves.
Once you need to rewrite everything, there’s no reason to stay with something you know since you need to fully retool anyway.
As a Perl programmer since v5 was released, the confusion around 6 completely destroyed almost everyone’s enthusiasm, and immediately caused all new projects to avoid Perl. It seemed like 5 had reached the end of the line, and 6 was nowhere to be found. Nobody wants to gamble so many hours of their lives, and the future of their business, on such an uncertain environment.
If Perl 6 had any visible movement within the first few years, it might have survived, but it was a good decade before they even admitted Perl 6 might take longer than expected, and then more time after that before they admitted it should have been a new language. 6 was interesting for language geeks, and they probably did some cool things, but you can’t run a large popular project like it’s a small research project. That completely destroyed all momentum in the community. Perl 5 development only resumed far too late, after the writing was already on the wall.
Both Bill Gates and Linus understand backwards compatibility as a sacrosanct principle. Python only just barely survived the jump from 2 to 3. JavaScript can only survive this because there’s no other option in a browser.
I learned Python from reading a pocket language reference that just described the syntax and standard library, because the language was simple and easy to understand and everything made sense.
Conversely, I was trying to debug a script someone else ran and came across a line that said '$|++'; it was impossible to search for on the web, and when I asked on IRC the only answer I got was 'man perldoc' which also did not answer my question in any reasonable way.
For anyone wondering: `$|` is an alias for `$OUTPUT_AUTOFLUSH`; it defaults to 0 (line-buffered) but any non-zero value means 'flush output immediately'. Thus '$|++' changes 0 to 1 (or 1 to 2, etc), which means that '$|++' means 'turn off output buffering'. No one could be bothered to say that; if you had questions about the language you clearly didn't RTFM well enough so that became the default/only answer I ever saw.
Meanwhile, the PHP community was often welcoming and helpful to newcomers, despite most of them being bad at programming and giving bad advice, and the Python community produced a language that was so often self-explanatory that new user questions were more about how Python did things or asking about how to implement things they didn't realize were in the standard library.
So yeah, lots of things contributed to Perl's decline, but the community being a bunch of elitist toxic dicks sure didn't help matters and it meant that as the set of people looking to learn how to do programming on Linux grew past the neckbeards looking for any metric to show that they were better than other people then Perl's growth potential was finite.
Perl had and still has truly AMAZING documentation (and far better in perl 5 than Perl 6 - where the documentation's search function is regularly broken for example). And out of this world bring-up tutorials. But if the bar is at "I want to be up to speed after reading a short booklet" - well, that's not gonna work. For me I have programmed in perl for decades and I still get the occasional deep plunge in a feature area that I never used before... And the various docs and books still come through.
On top of that, Perl 5 had an amazing community ready to help - but understandably annoyed with constantly re-quoting the AMAZING documentation. It could have gone either way, it just went the way it did: experts present and ready to answer. But there too - if you expect the community to constantly answer the most basic questions, well, Perl was not for you.
As a tradeoff perhaps, that expert community also spent a lot of effort on the AMAZING module library.
Perl 5's thorough documentation, plus splendid course book, plus all the books, plus CPAN, PLUS the expert help producing fantastic rabbit holes to learn more about your PROFESSIONAL MEDIUM. THAT was the reason for choosing perl.
If anything, newcomers don't go to the documentation because they are not aware that documentation can be THAT good. That was, still is, a marketing error.