Thanks for your input. I guess I should have given more description. I tried

if ( $str_read_buf =~ m/^\X/ ) {
    # do success stuff
}
else {
    # fail stuff
    # processing never gets here
}
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, but an error was never detected. The m/\X/ detects the invalid byte, because it returns displays the malformed character message, but did not fail. I should have mentioned that this is version 5.6.1.