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*

Well, the direct equivalent* of mine, above, would be:
(* other than scoping issues of $i)

  my $i = 0;
  while ( $i <= $#a ) {
      push @b, [ @a[ $i .. $i+$n-1 ] ];
  }
  continue {
      $i += $n;
  }
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Thing I don't like about yours is the incessant multiplication by $n.

jdporter
The 6th Rule of Perl Club is -- There is no Rule #6.