You probably ought to check the return values of sysread and syswrite. They both return undef on error: 
$result = syswrite(COM, $data);
if (defined $result) {
    print "$result bytes written\n";
} else {
    die "Couldn't write to port.\n";
}

[download]
Of ourse, there's always the CPAN module Win32::SerialPort.

In reply to Re: Accessing serial ports under Windows by chromatic
in thread Accessing serial ports under Windows by Odud

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