You're right. For a complete solution, I'd probably have done more. At the very least, I should have shown some code to walk the hash in some reasonable order to print the results.

foreach my $date (sort by_date keys %sums)
{
    print "$sums{$date}\n";
}
[download]

where by_date is a subroutine that can compare the two dates in $a and $b appropriately for sort.

For a more robust solution, you could also take the date values and convert them into some canonical form before using them as keys in the hash. (If that's easier to sort, so much the better.)

To start with, though, I really didn't want to generate a response that was too complicated to understand in the simple case.