my @message_lines = 
  ( '', # blank line
    'This is a multi-part message in MIME format.',
    '','',  # a couple blank lines
    "------=$_random_boundary",
    ...,  # i.e. one array element for every line of message content
    '',''  # including a couple blanks at the end
   );

my $_mail_message = join( "\r\n", @message_lines );