At blekko we're building a web-scale search engine in perl. It includes a distributed platform (kind of like bigtable) currently running on ~200 machines (soon to be 700). The total datastore, containing several n-billion page crawls and associated indexes on our cluster is about 350tb.

I think of perl as a replacement for C. We're using it as a system language, doing things like running epoll event loops, sending/receiving udp packets, mmapping files, etc.

IMO perl, python and ruby are all great choices for a project. Perl seems to be unfashionable these days, but we picked it because it came out the fastest on the benchmarks for what we're doing, and being the oldest is has some of the deepest library support through CPAN. The pragmatic-vs-idealistic attitude of the community is also a plus for us.