sub prepchksum {
    my ( $payload ) = @_;

###DEBUG:
    printf "BEFORE = %s\n", unpack 'H*', $payload;

    if ( length( $payload ) % 2 == 1 ) {
        if ( substr( $payload, -1 ) ge "\x80" ) {
            substr $payload, -1, 1, chr ord( substr $payload, -1 ) - 1
+;
            substr $payload, -1, 0, "\xff";
            }
        else {
            substr $payload, -1, 0, "\0";
            }
        }

###DEBUG:
    printf "AFTER  = %s\n", unpack 'H*', $payload;
    }
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