Perl's decline was cultural

Python 3 almost killed Python.

It's normal. Once a community loses faith, it's hard to stop them from leaving.

Perl was effectively "dead" before Perl 6 existed. I was there. I bought the books, wrote the code, hung out in #perl and followed the progress. I remember when Perl 6 was announced. I remember barely caring by that time, and I perceived that I was hardly alone. Everyone had moved on by then. At best, Perl 6 was seen as maybe Perl making a "come back."

Java, and (by extension) Windows, killed Perl.

Java promised portability. Java had a workable cross-platform GUI story (Swing). Java had a web story with JSP, Tomcat, Java applets, etc. Java had a plausible embedded and mobile story. Java wasn't wedded to the UNIX model, and at the time, Java's Windows implementation was as least as good as its non-Windows implementations, if not better. Java also had a development budget, a marketing budget, and the explicit blessing of several big tech giants of the time.

In the late 90's and early 2000's, Java just sucked the life out of almost everything else that wasn't a "systems" or legacy big-iron language. Perl was just another casualty of Java. Many of the things that mattered back then either seem silly today or have been solved with things other than Java, but at the time they were very compelling.

Could Perl have been saved? Maybe. The claims that Perl is difficult to learn or "write only" aren't true: Perl isn't the least bit difficult. Nearly every Perl programmer on Earth is self-taught, the documentation is excellent and Google has been able to answer any basic Perl question one might have for decades now. If Perl had somehow bent itself enough to make Windows a first-class platform, it would have helped a lot. If Perl had provided a low friction, batteries-included de facto standard web template and server integration solution, it would have helped a lot as well. If Perl had a serious cross-platform GUI story, that would helped a lot.

To the extent that the Perl "community" was somehow incapable of these things, we can call the death of Perl a phenomena of "culture." I, however, attribute the fall of Perl to the more mundane reason that Perl had no business model and no business advocates.