1. It pulled away folks who would otherwise have spent time improving Perl 5 (either the core or via modules).
2. It discouraged significant changes to the Perl 5 language, since many people figured that it wasn't worth it with Perl 6 just around the corner.
3. It confused CTO/VP Eng types, some of whom thought that they shouldn't invest in Perl 5, since Perl 6 was coming soon. I've heard multiple people in the Perl community discuss hearing this directly from execs.
Of course, hindsight is 20/20 and all that.
Also, even if Perl 6 had never happened the way it did and instead we'd just had smaller evolutions of the language in major versions, I think usage would still have shrunk over time.
A lot of people just dislike Perl's weird syntax and behavior. Many of those people were in a position to teach undergrads, and they chose to use Python and Java.
And other languages have improved a lot or been created in the past 20+ years. Java has gotten way better, as has Python. JavaScript went from "terribly browser-only language" to "much less terrible run anywhere language" with a huge ecosystem. And Go came along and provided an aggressively mediocre but very usable strongly typed language with super-fast builds and easy deploys.
Edit: Also PHP was a huge factor in displacing Perl for the quick and dirty web app on hosted services. It was super easy to deploy and ran way faster than Perl without mod_perl. Using mod_perl generally wasn't possible on shared hosting, which was very common back in the days before everyone got their own VM.
All of those things would still have eaten some of Perl's lunch.
But being the first meant it got some oddities and the abstractions are not quite right imho.
A bit too Shell-esque, specially for arguments passing and the memory abstractions are a bit too leaky regarding memory management (reference management fills too C-esque for an interpreted language, and the whole $ % @ & dance is really confusing for an occasional and bad Perl dev like me). The "10 ways to do it" also hurts it. It lead to a lack of consistency & almost per developer coding coding styles. The meme was Perl is a "write only language".
But I would still be grateful of what it brought and how influential it was (I jock from time to time how Ruby is kind of the "true" Perl 6, it even has flip flops!).
In truth, these days, I feel the whole "interpreted languages" class is on the decline, at least on the server. There are a lot of really great native languages that have come up within the last few years, enabled in large part by LLVM. And this trend doesn't seem over yet.
Languages like Rust, Swift, Go, Zig or Odin are making the value proposition of interpreted languages (lower perf but faster iterations) less compelling by being convenient enough while retaining performance. In short, we can now "have the cake and eat it too".
But the millions of lines in production are also not going awywhere anytime soon. I bet even Perl will still be around somewhere (distro tooling, glue scripts, build infra, etc...) when I retire.
Anyway, thank you Perl, thank you Larry Wall, love your quotes.