My guess is that it's because Perl is very comfortable for a team that mostly writes C on a Unix like platform.

The general syntax, operators, etc, are close, and familiar things like stat(), gethostbyname(), etc, work almost the same.

Edit: One of their Perl scripts for reference: https://github.com/openbsd/src/blob/b66614995ab119f75167daaa...

And pkg_add, which has quite a lot of Perl: https://github.com/openbsd/src/tree/master/usr.sbin/pkg_add