Open the 2nd file (the 3000 number file). Read the first eight digits of each entry into a hash, where the eight-digit number is a hash key.

Open the 3000000 number file. For each entry in that file, take the first eight digits, and check for the existence of a matching hash key. If one exists, increment the value indexed by that key.

Sort the keys by their corresponding value.

Print the first 100 entries from the sorted list of hash keys.

Start with this:

#!/usr/bin/perl 
# ...or whatever shebang line your system requires

use strict;
use warnings;

# Follow the steps outlined above...
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perlintro will contain everything you need for this. It takes about a half hour to read through it. After having read perlintro, and after getting started, you will probably come up with more specific questions. When asked along with code to illustrate your sticking points, we'll be able to help you along.