my %sum_of;
while (<DATA>) {
    my ($k, $v) = split /,/;
    $sum_of{$k} += $v;
}

print "$_,$sum_of{$_}\n" for sort keys %sum_of;

__DATA__
2010-02-12_aaa,654
2010-02-12_aaa,248
2010-02-12_bbb,374
2010-02-12_ccc,158
2010-02-13_aaa,745
2010-02-13_bbb,786
2010-02-13_bbb,354
2010-02-13_ddd,852
2010-02-14_bbb,754
2010-02-14_aaa,169
2010-02-14_ccc,965
2010-02-14_ccc,756