sub combinations {
  my @array = @_;
  if ($#array == 0) {return [ $array[0] ]; }
  my @results;
  my $element;
  foreach $element (0..$#array) {
    my @leftovers = @array;
    my $chosen_one = splice(@leftovers, $element, 1);
    foreach (&combinations(@leftovers)) {
      push(@results, [ $chosen_one, @{$_} ]);
    }
  }
  return @results;
}

use Data::Dumper;
print Dumper(&combinations(qw(a b c d)));