use strict;
use warnings;

my $amount = 16;
my $coinset = [5,2];

{
  my $Exception = 0;

  sub change {
    my ($cset, $amt) = @_;
    if ($amt == 0)    { return [] }
    if (@$cset == 0)  { $Exception = 1; return [] }
    my $coin = shift @$cset;
    if ($coin > $amt) {
      print "$coin > $amt\n";
      return [change($cset, $amt)];
    }
    else {
      print "Checking $amt - $coin\n";
      my $rval = [$coin, @{change( [$coin, @$cset], $amt - $coin )}];
      if ($Exception) {
        print "Exception forces backing up to $amt\n";
        $Exception = 0;
        $rval = [@{change($cset, $amt)}];
      }
      return $rval;
    }
  }
}

use Data::Dumper;
print Dumper(change($coinset, $amount)), "\n";