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#! perl -slw
use strict;
...
?9999FF1B? : 0
?9998FF1B? : 1

9999 = 57 + 57 + 57 + 57 = 228.
~ 228 = -229 decimal = FF1B hex (16-bit)
Add the header and trailer characters = <SOH>9999FF1B<ETX>

1 + 57 + 57 + 57 + 57 = 229

FF1B = 65307

(65307 + 229) = 65536
65536 & 65535 = 0x10000 & 0xFFFF = 0

sub build_string {
    my( $string ) = @_;
...

    return chr(1) . $string . sprintf( '%4x', $chksum ) . chr(3);
}