If it involves a lot of text processing, Perl tends to be either my first choice or pretty close to it. Perl's also surprisingly fast for an interpreted scripting language (especially when it comes to text processing). I've used it pretty heavily for system automation tasks, too, and thus generally continue to do so unless there's some specific reason why I'd need to use a different language (and usually there ain't).

I've heard that Perl's also massively popular in the genetics/bioinformatics world. I'm assuming a lot of their data is in textual formats, and thus would benefit from a language that's fast at processing that data. I have no experience here, though.