Perl's decline was cultural

Perl's decline was cultural in the same way VMS's decline was. A fantastic approach and ecosystem—just one overtaken by a world that was moving on to a different set of desires and values.

PHP emerged as a separate language and community and "ate Perl's lunch" when it came to the dominant growing app style of the Aughties... web pages and apps. Had PHP instead been a Rails-like extension of Perl for the web, sigils would have reigned for many years more. But there was never a WordPress, Drupal, or similar in Perl, and no reason for anyone who wasn't already highly connected to the Unix / sysadmin community to really give Perl another look.

By 2005 if you weren't already deep into Perl, you were likely being pulled away to other communities. If you were into Perl, you were constantly lamenting why newbies and web devs weren't using your language, just like the DECUS and VMS crowd did a decade earlier as Unix and Windows consumed all the oxygen and growth opportunities in the room.

VMS decline was bound to a failing hardware business and company. That's a very different thing. Unix was around in the 80s and VMS did fine, its when DEC hardware business went down the tubes VMS lost out big time.