The trick is to use a hash:

use strict;
use warnings;

my $file = 'msgcount.txt';
open(LOG, '<', $file) or die "Can't read '$file': $!";

my %sum;
while (<LOG>){
    chomp; # remove newline
    my ($date, $name, $number) = split m/ /, $_;
    $sum{$name} += $number;
}

# now print the result:
while (my ($name, $total) = each %sum){
   print "$name\t$total\n";
}
[download]