sub unordered_sets {
  my ($in, $out, $left, $idx, $curr) = @_;
  push @$out, [@$curr] and return if $left == 0;
  $idx ||= 0;
  for ($idx .. $#$in) {
    push @$curr, $in->[$_];
    unordered_sets($in, $out, $left-1, $_, $curr);
    pop @$curr;
  }
}

my @sets;
unordered_sets [1..6], \@sets, 6;