First method (non-Perl):

1. sort fileA and write it to /tmp
2. sort fileB and write it to /tmp
3. Use cmp or diff to detect the differences between the two /tmp files and report.

1. read fileA and use each value as a hash-key with a hash-value of 1 ($seen_hash{$valueA} = 1;)
2. read fileB and use each value as a hash-key and add a value of 2 ($seen_hash{$valueB} += 2;)
3. Read through the hash and report on all keys where the value is '3'.