use DBIx::Simple;

my $db = DBIx::Simple->connect("...",{RaiseError => 1} ) or die "can't\n";

my $cust_result = $db->query('SELECT cust_ID, cust_name FROM customer');

while (my $cust = $cust_result->hash) {
    my $orders_result = $db->query(
        'SELECT * FROM orders WHERE cust_ID = ?',
        $cust->{cust_ID}
    );

    while (my $order = $orders_result->hash) {
        # code
    }
}