You have to show us what $pbxex looks like. If the hash already has something in it, your code seems to work fine for me:

use warnings;
use strict;

my %hash = (CS => 4);
while (<DATA>) {
    chomp;
    $hash{$_}++;
    print "$_$hash{$_}\n";
}

__DATA__
CS
CS
[download]

prints:

CS5
CS6
[download]