First off, you are probably limiting your audience unnecessarily by mentioning "threads" in the title.
Your basic problem, that of needing a non-blocking read, has nothing to do with threading.
Secondly, do you really need to terminate your read thread? That is, if you detached it, it would just silently melt into the ether when your main thread decides to exit the process.
But if you do really need to, I think that you've already discovered the 'right' solution, namely read_bg(). All you nee to do is use the API correctly and alter your coding strategy a little. (NOTE: None of the following is tested.)
So I changed to "background read" my ($receivedBytes, $data) = $HandleToRS232->read_bg(); Doing this I got nothing in $data so I did a call to my($done, $count_in, $string_in) = $HandleToRS232->read_done(1); Then I receive the message in the $string_in variable, but the call to read_done is also blocking.
Based on my reading the source -- and I may well have misread it --
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