If you don't mind building up a hash into memory every time you run it, and you don't mind not only having to hardcode the service, but also having to use the form '21/tcp' instead of '21tcp', then the following code should do the job:

perl -lane'$h{$F[1]}=$F[0]}{print $h{"21/tcp"}' /etc/services
[download]

I would be surprised if there is not an existing CPAN module for this task ... i don't know of one off hand ...

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)