I would do it recursively:
use warnings;
@denoms = qw(2 5 10 20 50 100);
sub change {
my ($unchanged, $nothing_smaller_than) = (@_,0);
my @results = ();
for my $trybill (@denoms) {
next if ($trybill < $nothing_smaller_than);
if ($trybill < $unchanged) {
my $remaining = $unchanged - $trybill;
push @results, map {[$trybill, @$_]} change($remaining, $trybill
+);
}
elsif ($trybill == $unchanged) {
push @results, [$trybill];
}
else { last }
}
return @results;
}
use Data::Dumper;
print Dumper change(100);
Try each denomination against what is to be changed. If the bill being tried ($trybill) is less than the amount to be changed ($unchanged), then make a result set out of $trybill mapped to each combination returned by change($unchanged-$trybill).
The $nothing_smaller_than parameter ensures that the output doesn't result in multiple permutations of the same combination of bills: only the canonical order of smallest bill to largest bill will be returned.
Caution: Contents may have been coded under pressure.