Migrating hundreds of thousands of email accounts from one system to a new one. Source information was variously LDAP, custom information systems, Lotus crap, etc etc. Destination was another LDAP server. Perl took all that text in, munged it around, then loaded it into the destination. Back then I didn't know it was called ETL, and to me it was like magic that I had written.
Out of all the ETL systems and languages I've encountered over the years since then, Perl has been the fastest to develop and most flexible. The "text in, regex or otherwise select into objects, transform stuff, then transmit to server" abilities of Perl were just so slick.
Also, Perl is partly responsible for my move to the USA. Slashdot and its associated Everything2 site were written in Perl, and I met my (now-ex) wife through Everything2.
Thank you, Perl, for being pretty danged awesome and oh so flexible. :)
EDIT: and thank you Larry Wall for showing me that programming doesn't have to be stodgy and boring, that language has beauty.