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$field_start  = 8;  # 0-based
$field_length = 8;
...
$new_value = 'bla';
substr($s, $field_start, $field_length) =
   substr($new_value . (' 'x$field_length), 0, $field_length);

foreach (@old_records)
{
...
      sprintf("%-7s %-13s %s\n", $name, $desc, $date)
   );
}