What makes Perl interesting to me is the philosophy behind the language design. "Since Perl was designed (loosely speaking) by an occasional linguist, it was designed to work smoothly in the same ways that natural language works smoothly." (1)
The three mantras that I remember from my days writing Perl are: "tim-toady" (There's More Than One Way to Do It); "Easy things should be easy, while hard things should be hard;" and (borrowed from Voltaire) "don't let perfect be the enemy of the good."
Perl is messy, because language is a messy business. Perl is expressive, in part because it is messy. Perl is fun to write, and if you have the spare cycles it's worth your time to explore, purely for the mental calisthenics.
:-)
"Easy things should be easy and hard things should be possible"
Please don't link to illegal copies of books. (Besides, the fourth edition of "Programming Perl" is released.) Chromatic (the author of the linked article) has a very good and recent book about modern Perl instead, with a pdf online.
The difference is in their niche. Perl continues to retain its niche on the CPAN end- To provide solutions to nearly every problem you are likely to face. Its also very easy to rapidly prototype with Perl, you can build things very quickly, there is tons of documentation, help and nearly all your day to day problems have been solved and answered somewhere or the other.
Unicode, CPAN, Regular Expression, Functional Programming, Text Processing, Data Processing, Large Data Munging et al continue to remain Perl's core strengths. You also have good web frameworks these days.
If you have a little time for learning great programming stuff you must take time to read Higher Order Perl by Mark Jason Dominus which is a free book.