sub my_make_change {
  my $amount = shift;
  my $coin = shift || -1;
  return(
    map {
       my $n = $_;
       map [ [ $n, $coin ], @$_ ],
           ($n*$coin == $amount
            ? []
            : my_make_change( $amount - $n*$coin, @_ ));
    } (0..$amount/$coin)
  );
}