Let's back up a minute and think about the problem.

To do the comparison, you need
- the file to be compared (file_a)
- the file to compare with (file_b)
- a method of doing the comparison

Given two lists of files (@file_a) and (@file_b), then you just need to take one from column a and one from column b, apply the comparision method, and check the result. For example you could:

$fa = shift(@file_a);
$fb = shift(@file_b);
if (digest($fa) == digest($fb)) # digest=whatever method you prefer
                                # use eq instead of == if digest retur
+ns a string
{
    # files match
}
[download]

So you have two separate problems: first generating the two lists of files so that the appropriate files match up in each list, then doing the proper digest.