# untested but realistic
my $dbh = DBI->connect("...",{RaiseError => 1} ) or die "can't\n";

my $cust_sth = $dbh->prepare(
    qq{SELECT cust_ID, cust_name from customer});

$cust_sth->execute();

while (my $cust = $cust_sth->fetchrow_hashref()) {
   my $orders_sth = $dbh->prepare(
        qq{select * from orders where cust_ID = ? });
   # do something smart with customer name

   $order_sth->execute($cust->{cust_ID});

   while (my $order = $orders_sth->fetchrow_hashref()) {
       # do something even smarter with the orders data
   }
}

##</code><code>##

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
    }
}

##</code><code>##

use DBIx::Simple;

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

for my $cust ($db->query('SELECT cust_ID, cust_name FROM customer')->hashes) {
    for my $order ($db->query('SELECT * FROM orders WHERE cust_ID = ?', $cust->{cust_ID}->hashes) {
        # code
    }
}