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.)
Much like in English, you can use Perl's for()for iterating over a list, iterating via initialization + check + step, or associating a single topic with a block of syntax. So I, without apology, use for()for topicalizing. For you, I won't stop doing this. (: Excuse me for not demonstrating the use of English "for" analogous to init + check + step.