my $serial_port = start Win32::SerialPort('pump.cfg');
$serial_port->write_settings;

$serial_port->write("aU\n");
sleep(1); # Same result w/ and w/o sleep here too
while (1) { 
    my ($rb, $byte) = $serial_port->read(1);
    if ($rb > 0) {
        print $byte;
    } else {
        last;
    }
}