$SQL = "select name from customers where 1=1";

if ($fl_have_cust_id) {
   $SQL .= " and custid=?";
   push @args, $cust_id;
}

if ($fl_have_sales_id) {
   $SQL .= " and salesid=?";
   push @args, $sales_id;
}

$ST = $DB->prepare($SQL);
$ST->execute(@args);