Maybe something like this?

#!/usr/bin/perl -w
use strict;
use Data::Dumper::Simple;

my %codes;

while (<DATA>) {
    chomp;
    my $foo = (split /:/, $_)[1];
    if (!$codes{$foo}) {
        $codes{$foo}++;
    }
}

print Dumper(%codes);

__DATA__
HIT: code2
HIT: code3
HIT: code1
HIT: code90
HIT: code2
HIT: code34
HIT: code90
[download]

Which prints:

%codes = (
           ' code90' => 1,
           ' code2' => 1,
           ' code3' => 1,
           ' code2 ' => 1,
           ' code1' => 1,
           ' code34' => 1
         );
[download]

Actually, you don't really need the "if" block inside the while loop. You could just increment each hash value regardless. That won't give you any duplicates - it will just mean that some have a higher numerical value. Which (if I understand correctly) shouldn't make any difference.

Hope this helps,
Darren :)