One way is to store the data in a hash-of-hashes: 
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

[download]

prints: 

A: sum=46 avg=11.5
B: sum=31 avg=15.5
C: sum=17 avg=17
D: sum=18 avg=18

[download]

In reply to Re: get the average of hash values? by toolic
in thread get the average of hash values? by luckysing

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