$ perl -E '/(?{ say "See?" })/'
See?

##</code><code>##

$ perl -E 'use re "eval"; my $a = qr//; /(?{ say("See?") })$a/'
Undefined subroutine &main::say called at (re_eval 1) line 1.

##</code><code>##

$ perl -E 'use re "eval"; my $a = qr//; /(?{ use 5.010; say "See?" })$a/'
See?