Untested:

my %data;
while (<DATA>) {
    next if /^ID/ || /^\s$/;
    my ($id, $value) = split /,/;
    $data{$id}[0] += $value;
    $data{$id}[1] ++;
}
my %avg;
while (my ($id, $info) = each %data) {
    $avg{$id} = $$info[0] / $$info [1];
}
[download]