Perl is an acronym for Practical Extraction and Report Language, but it's most commonly spelled as a proper name. It's a very popular and powerful scripting language used for web applications, although its use is being largely replaced by PHP in the mainstream. One of its strengths lies in its speedy and effective use of regular expressions. Its unofficial motto is, "There's More Than One Way To Do It," or "TMTOWTDI," owing to the extreme flexibility of the syntax.
WordPress does not use Perl, and it is therefore not required.