I personally believe I may come out as an annoying little voice, but I have to join the chorus: that is a statement-modifier-for which happens to have the form of Perl-style C<for> acting on $_ - and saving you the need of parens. But by a (generic) Perl-style C<for> it is meant either:

for my $i (LIST) { ... }  # or
for (LIST) { ... }
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"C-style" is in no way bad "because of useless $i variable" since indeed there may not be any $i variable at all: it's "just" a completely different kind of loop altoghether (a more generic one - and that is fundamentally the reason why it exists at all,) which bears the same name as Perl-style one due to the fact that it can easily be disambiguated from the latter by means of the syntax of what's in the parens. I would go as far as claiming that what's in the parens itself in that case is yet another minilanguage (very closely related to Perl itself: it consists of exactly three Perl statements!) that does not fit in other parts of Perl's syntax, and thus a very special case: of course Perl 6 knows better and gives it an entirely different name which IIRC is C<loop> and not by chance takes one letter more wrt "for" since that's for huffmanization and in fact it is thought to be on a much sparser basis to begin with.