To aid in my irrational goal to never use the C-style for loop in Perl -- as well as the more widely accepted goal of TMTOWTDI -- here's how I'd do that:

my @a = qw(a b c d e f g);
my $n = 3;

my @b;
for (0 .. $#a/$n) {
  push @b, [ @a[$_*$n .. $_*$n+$n-1] ];
}
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I was going to make the case that I like to avoid the C-style for loop because it looks ugly in Perl with all the sigils clogging things up, but after I typed out my alternative, I realized its readability is not much better. *shrug*