Perl is a portable general-purpose dynamic programming language. It is used in a diverse variety of domains: system management scripting, text processing, web content management, Biology calculations, and others.
Perl has a rich library of free reusable modules called "CPAN", and a vibrant community of users and developers.
Perl's original description from the Perl documentation:
Perl is a language optimized for scanning arbitrary text files, extracting information from those text files, and printing reports based on that information. It's also a good language for many system management tasks. The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal).
The CPAN is Perl's module repository and is where users should turn first for code that implements solutions to already-solved problems.