Forcing Mail::Message to be multipart/alternative instead of multipart/mixed

I have a script which is dissecting a multipart eMail into it's
sub parts

<snip>
  my $CounterII=0;
  foreach my $Parts ($Message->body->parts) {
    $Part[$CounterII] = $Parts->decoded->string;
    $Type[$CounterII] = $Parts->get('Content-Type');
    $CounterII++;
 }
<snip>

After doing some editing of the parts I want to put them back
together again, in a multipart/alternative message, not the
default multipart/mixed format

My thought shown here is not the answer..

<snip>
 my $SendMessage = Mail::Message->build(
   Subject=>$Message->get('Subject'),
   From =>$NewFromAddress,
   To => $NewToAddress,
   mime_type => 'multipart/alternative',
   data => $Part[0],
   attach=>@Bodies
 ); 

Does anyone know the correct way to do this?

Zaphod