You could sum as you go, or you could sum afterwards:

use strict;
use warnings;
use List::Util;

my %yahoo = ();

while (<DATA>) {
    my ($value,$key) = split;
    push @{$yahoo{$key}}, $value;
}

foreach my $key (sort keys %yahoo) {
    my $sum = List::Util::sum (@{$yahoo{$key}});
    print "$key: $sum\n";
}

__DATA__
12   AAAA
12   AAAA
15   BBBB
1222 CCCC
1    BBBB
[download]

Prints:

AAAA: 24
BBBB: 16
CCCC: 1222
[download]