I have no idea what your code is trying to accomplish - if you want to add things together, there should at least be a + somewhere, but I guess you want all the sums in a hash, right?

use warnings;
use strict;

my %sum;
while(my($id, $value) = split ',', <IN>) {
    $sum{$id} += $value;
}
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Please do remember to use both strict and warnings. It will save you a lot of grievances.