sub perms {
  my $arr = shift;
  if ( !@$arr ) {
    return ( [] );
  }
  else {
    return map { my @a = @$arr;
                 my $e = splice @a, $_, 1;
                 map [ $e, @$_ ], perms( \@a ) } ( 0..$#$arr );
  }
}