my $name = 'bob';
my $amount = 110;
$sum{ $name } += $amount; ## sum of bob is increasing
$amount = 100;
$sum{ $name } += $amount; ## sum of bob is increasing
$amount = 10;
$sum{ $name } += $amount; ## sum of bob is increasing
$amount = -100;
$sum{ $name } += $amount; ## sum of bob is decreasing