Why is Perl the most hated language?

I don't know why it's the most hated language but I can venture a guess. Perl fostered a "there is more than one way to do it" philosophy from the beginning, which let to some developers trying out clever tricks to do simple tasks.

That often leads to code which is hard to maintain in the long run, especially when code changes hands. Code is read much more often than it is read, so people inheriting large code bases written in Perl have often found it hard to maintain or extend existing code when they encounter these tricks.

This may be one reason. Of course, with discipline and a sane set of  standards on a project, Perl can probably be written as well as any other language.