Start with this:

#!/usr/bin/perl
use strict;
use warnings;
#psudo code from here on
# open the file with categories 
# read the categories in (probably to a hash where the key is the cate
+gory and the value 0)
# close the categories file (you have your hash, you don't need to rea
+d from the file anymore)
# open the file with 3000k entries
# read the file line by line
# for each line read
#    trim to the first 8 characters
#    look for that value in the hash keys
#    increment the value of the hash key that is matched if any
# you now have a hash with category as the key, and the number found a
+s the value
# you should be able to figure out how to find the top 100 values and 
+print out the key and value for each of them or store them to file
[download]

Run the program using the -d (debugger) and learn to use that tool to examine and learn what those hashes and any other variables look like. It is a quick tool to learn to use if just doing basic examining for self-enlightenment.

PerlDoc is your friend. Tutorials like perldsc, perlop, perlfunc will all help you solve this pretty quickly, including example code much of the time.

Hope you find this helpful... Update:

Note that by using a hash, you eliminate the possibility of there being duplicate categories, simplifying and possibly making the effort more efficient.

Restated the increment step for clarity(I hope)