You can write readable Perl, just like you can write secure PHP. The fact that a large number of people don't is not necessarily a failing of the language.
Readability and reasonable shoot-self-in-foot protection are not optional. See the article from earlier today about not catering to power users. Same concept. You can't really design a language for the top 5% of programmers and expect it to gain any sort of acceptance. (See: Haskell, Ocaml)
Perl is a power tool. The reason it can be unreadable is because of it's terse syntax, wherein resides its power. If you value readability above power then you choose Java, and I would agree that for a large enterprise app developed by an army of mediocre developers then Java is surely the better choice. However to judge the absolute merits of a language like Perl on reputed readability, without taking into consideration what its terseness allows, and how it works for large scale development given a sane coding standard is just burying your head in the sand.
You will be shocked how few tools are there for the other kind of data.
Basically - it's a balance - don't be too dumb, but don't be too clever either. I'd submit Python as being solidly in the middle ground here.
Your average PHP web dev is not the cutoff for the 95th percentile. But yes, I can imagine them trying to understand monads. I have also observed them succeeding at understanding monads. They aren't hard to learn, just something you need to learn. Being able to learn simple things doesn't make you the top 5%.