...
my $msg = MIME::Lite::TT::HTML->new( 
            From        =>  'ex@why.com',
            To          =>  $params{'emailto'}, 
            Subject     =>  'Your recent purchase', 
            Template    =>  {
                                html    =>  'test.html.tt',
                            },
            TmplOptions =>  \%options, 
            TmplParams  =>  \%params, 
 ); 
$msg->attr("content-type" => "multipart/mixed");
...