I don't have Device::SerialPort on my machine, but did you try different values for flow control? You said that the appliance expected no flow control, but you set the serial port in your script to RTS. Hopefully it's as simple as setting flow control in your script to "none".

Update: I installed Device::SerialPort and ran your script with minicom running on the other end of the cable and it worked as expected, so I'm going to stick with my guess that you have one of the communication parameters mismatched.