One criticism of opponents of Perl is that it is a "write-only" language meaning that once the code is written, it is extremely difficult to maintain because it is difficult to understand upon re-examination. As with many criticisms, this should be aimed at those undisciplined developers who are writing the code, and not their tool of choice.
Having said that, I think it is also fair to say that Perl makes it very easy to write difficult-to-decipher code. This is the double-edged sword which is the shorthand Perl gives us to be very expressive in a small amount of space. A negative application of this is obfuscated Perl (where the author intentionally makes his code difficult to read), while a more positive application is the craft of creating Perl "oneliners" (trying to include a great deal of functionality in a single line of code). A oneliner can be a powerful weapon in the arsenal of a system administrator.