I used it in probably one of the more cursed contexts I've ever heard of. Understand[0] is a static analyzer for many languages, and one of its killer features is that it is programmable with a Perl API. I used this feature at a defense consulting job to help target audits of huge, multi-million LOC codebases.
Perl's expressivity was very useful here. I cut my teeth on functional programming concepts to write some very nice traversals. The runtime environment of the host program was a nightmare to deal with though.