I read the MIME RFC for a little something I'm working
on ( I should really start working on it again :-).
I think this really cheap code will help you:

use MIME::Base64;

$smtp->data();
$smtp->datasend( "\nSubject: $subj \nMIME-Version: 1.0" );
$smtp->datasend( "\nContent-Type: multipart/mixed;boundary=\"Bb0uNddAr
+ray\"" );
$smtp->datasend( "\n--Bb0uNddArray" );
$smtp->datasend( "\nContent-type: text/plain" );
$smtp->datasend( "\n\n$msg\n\n" );    # plaintext body
 
if( $attachement )
{

   $smtp->datasend( "--Bb0uNddArray" );
   $smtp->datasend( "\nContent-Type: $content_type; name=\"$filename\"
+" ); 

  $smtp->datasend( "\nContent-Transfer-Encoding: base64\n\n" );

      while ( read( $file, $buff, 512 ) 
       {
    $smtp->datasend( encode_base64($buff) ) || die"cant encode: $!";
       }
   
} # if attachement

$smtp->datasend( "\n\n--Bb0uNddArray--" );    # send this even if ther
+es no attachment    
$smtp->dataend();
$smtp->quit;
[download]

This is untested,
$smtp is a Net::SMTP handle

Hope it helps
-bl0rf