When you do

%customer_domain_hash = split(/@@@/, $line);

you are overwriting the entire hash every time whereas what really ought to happen is that only one entry should be added/changed. So, if you replace that one line with:

my ($key, $value) = split(/@@@/, $line);
$customer_domain_hash{$key} = $value;
[download]

it should build up the hash on each iteration of the foreach.

Update: Unless you have duplicate keys, of course, which has been pointed out below (thanks, GotToBTru). A simple hash is not sufficient to store this. toolic's answer shows an alternative approach.