#!/usr/bin/perl

use strict;
use warnings;

my %sum;
my %count;

for my $row (<DATA>) {
    chomp($row);
    my ($string, $zahl) = split /\s+/, $row;
    $sum{$string} += $zahl;
    $count{$string} ++;
}

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

__DATA__
foo 40
bar 41
baz 43
foo 44