the "Old" that appears above the text area is permanently zero length

Your form doesn't have a field/widget named "OldBugText" (neither hidden nor otherwise), and setting param(-name => "OldBugText", ...) doesn't magically create one.  As a result, the browser doesn't send a key-value pair for OldBugText to the CGI script (because it never got it itself as part of the form).  So when you try to print "Old: " . param('OldBugText'), the parameter is empty.

See Creating a Hidden Field.