It's called "given" in Perl 6 (or in Perl6::Switch, if you want to play with 6ish topicalizers in Perl 5).

Perl 6 also has syntactic relief for the m/\G.../gc monstrosity as well. That turns into m:p/.../, where the :p tells it to start matching at the current position. (But generally you don't even need that since subrules in a grammar always anchor to the current position anyway.)