use MIME::Lite;
my $msg = MIME::Lite->new(Type => 'multipart/alternative');
$msg->attach(Type => 'text/plain', Data => $msg_text);
$msg->attach(Type => 'text/html', Data => $msg_html, Encoding = 'quoted-printable');
$msg->scrub;
$msg_mime = $msg->as_string;