I'm having a bugger of a time trying to get the 'ORDER BY' bit to change from DESC (take as default) to ASC.

In other words, in the fragment

  if ($asc_desc eq 'ASC') {
    $sql .= " ORDER BY $order_by ASC ";
  } 
  else {
    $sql .= " ORDER BY $order_by DESC ";
  }
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you're always ending up with DESC. This suggests that $asc_desc isn't 'ASC', which you should be able to verify pretty quickly be viewing source for the page you're emitting, since you're putting out $asc_desc in a hidden field.

What RDBMS are you using? If you're using MySQL, 'ASC' is the default sort order, and you only add DESC to alter the order.