Practical Extract and Report Language
Perl is a modern high level scripting language written by Larry Wall. Most modern Unix and Linux systems come with it as standard, and many key distro applications are often written using it. Perl is a scripting language with a shell/c heritage, that is designed to get things done. Over the years it has evolved into a very powerful object-oriented language, and while complex applications can be written in it, it is not really designed to be used for time or memory critical applications, as Perl trades CPU and RAM for ease of use. Though the often quoted, Perl doesn't in fact stand for "Practical Extract and Report Language", it is just Perl, the name Pearl having already been taken!
Perl is often used as a glue language like shell, as it is very good at stringing together other applications, changing it's behaviour based on the output of the other application. It includes a very powerful Regular Expression engine, and excellent text manipulation tools. Perl is often called "The Swiss army Chainsaw".
Its text manipulation abilities, combined with its almost impossibility to suffer from buffer overrun problems; makes it a good choice for web CGI applications. Though CGI is slow, and the Perl interpretor is large and hence slow to load, Perl is still very popular for CGI applications, though for heavy usage mod_perl is better suited because the Perl interpretor is embedded into the Apache web server application. Perl has also been described as "The Duct Tape Of The Internet".
Perl has a very active user community, and is quite friendly.
http://www.perlmonks.org/ - A friendly online Perl community, newbie friendly
http://use.perl.org/ - A Perl News and Blogging site
http://www.perl.com/ - O'Reilly's Perl web site, news articles and more bloggs. Many of the better Perl books are ORA's
http://learn.perl.org/ - A site dedicated to learning Perl
http://www.cpan.org/ - A MASSIVE online repository of reusable Perl components (modules) very useful indeed. Some would say that CPAN is the strongest feature of Perl