If I send a message with some non ASCII Latin1 (well, windows1252) characters,

You can't do that given $mail{'Content-type'} = 'text/plain; charset="utf-8"';. It would be like Verizon quoting a price of 0.002 *cents* per kilobyte but charging you 0.002 *dollars* per kilobyte. (story) You can't tell the client you're using one encoding and but actually use another.

If you use $mail{'Content-type'} = 'text/plain; charset="UTF-8"';, then you'd use binmode($s, ":encoding(UTF-8)"); or encode("UTF-8", $text).

If you use $mail{'Content-type'} = 'text/plain; charset="cp1252"';, then you'd use binmode($s, ":encoding(cp1252)"); or encode("cp1252", $text).

Though I'm afraid of what it would do if someone did the encode('utf8', ...) on the text before turning it to Mail::Sender :-(

It would produce junk. You can't use both encode($encoding, ) and binmode(, ":encoding($encoding)").