Perl has stagnated, it's as simple as that. There are so many alternatives out there that have picked up where Perl left off or gone in new directions and covered some of the same ground that the competitive value of Perl has diminished over time.

I consider Ruby, for example, a more evolved Perl in a lot of ways.

Perl used to be the go to language for scripting and automation (practically what it was designed for) but now that niche is also filled with Python, Ruby, node, power shell, and improved automation tools like puppet, chef, or ansible.

Perl also used to be the go to language for dynamic web applications but it fell behind as other languages gained ground and especially as sophisticated frameworks like rails put Perl development to more and more of a comparative disadvantage.