The code

*MIME::Lite::send_by_smtp_tls = sub {
...
}
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installs the subroutine send_by_smtp_tls into MIME::Lite. I guess that it is all you need to later call

$mail->send_by_smtp_tls();
[download]

But as you left out the important part of it all, the code for send_by_smtp_tls, I don't know. Maybe the usage is

MIME::Lite->send('smtp_tls', $mail_host, Timeout=>60);
$msg->send;
[download]

instead.