use strict;
use warnings;

my %data;
while (<DATA>) {
    chomp;
    my ($id, $value) = split /,/;
    $data{$id}{sum} += $value;
    $data{$id}{count}++;
}

for my $id (sort keys %data) {
    my $avg = $data{$id}{sum}/$data{$id}{count};
    print "$id: sum=$data{$id}{sum} avg=$avg\n";
}

__DATA__
A,10
A,11
A,12
A,13
B,15
B,16
C,17
D,18