Perl and Python were similarly powerful and useful languages, but I could learn and start producing useful code in Python after reading an hour long tutorial. Perl took an order of magnitude longer, and remained more awkward to use just due to the Weirdness. There was a momentum building in the early 2000s toward competitors like Python and Ruby that were seen as less crufty and more modern.
Perl's developers seemed to agree, since they cooked up their own competitor to Perl, an entirely different language confusingly called Perl 6. The coexistence of Perl 5 and 6 made the Python 3 transition look like a cakewalk -- at least it would have save for Perl 6's almost entire failure to exist for over a decade after its inception. It produced lots of constantly churning specs and blog posts about register based virtual machines with native support for continuations or whatever, but no implementation of a language that anyone felt comfortable using for any real development. Meanwhile people kept using the ossifying Perl 5 for existing applications, and gradually transitioning away as they were replaced.
Also PHP overtook it for the "just FTP a script to $5 shared hosting and make a webapp" use case.
Perl is most human-like programming language, which is what makes it great, but also what makes it unreliable for large scale programming.