if (defined $total {$id}) {
    $total {$id} += $amt;
}
else {
    $total {tocredit} += $amt;
}  

##</code><code>##

$total {exists $total {$id} ? $id : "tocredit"} += $amt;

##</code><code>##

(exists $total {$id} ? $total {$id} : $total {tocredit}) += $amt;