# mark members with no referrals as done, an everyone else as not done.
# NOTE: I'm assuming a new column named is_done as a work column.  If you prefer
# not to have such a column in your database, you can use a temporary table instead.
$DB->do("update registered_users set is_done=0")
$DB->do("update registered_users set is_done=1 where MemberID not in (select referrer_id from registered_users");

while (1) {
   # Do we have any members that aren't done yet?
   my @tmp = $DB->selectrow_array("select count(*) from registered_users where is_done=0");
   last unless $tmp[0] > 0;

   # Perform computation for all members whose referrals are all "done"
   $DB->do(qq{
       UPDATE registered_users
       SET    is_done=1,
              <<<your calculation goes here>>>
       FROM registered_users
       WHERE is_done=0
       AND MemberID not in (
           SELECT referrer_id WHERE is_done=0
       )
   });
}