Any language can be unreadable.

Perl's gets a bad rap because it has concepts that don't exist in other languages (like context), and it has a very flexible syntax. If you want to write mud, Perl makes it easy. The good news is that Perl also makes it easy to write clean, maintainable, expressive code.

To avoid writing hard to read code, hang out here and absorb some coding style tips. Also, read Perl Best Practices--not everything in PBP is gospel, but it has tons of great advice and is well worth the time and price. Super Search for "Perl Best Practices" to find loads of good discussion on the recommendations in the book.

If you want to write truly scary code, definitely look into Obfuscation. I believe there is an obfu tutorial or two floating around the monastery. Acme::Pony is an inspired effort in this area.