It works nearly in the same exact way as the code you already asked about except that:

1. the flow control is syntactically (but not logically!) different;
2. it doesn't do a check on the actual strings, but on a checksum computed for them which gives a sufficient condition for two of them to be different. That is, if the checksums are different, then the strings will be different too, while the converse does not hold: different strings may have the same checksums (but we rely on the confidence that such occurrencies are rare enough), hence => my remarks

Then you wrote:

For example, I don't understand this line:

 my (@lines, %line_md5);
[download]

Oh my! Please tell me you're joking!! I see that you have 204 writing as of now. That's quite surprising... maybe you're not really programming in Perl, but in some vaguely similar language. What is it precisely that you do not understand?