Again, try actually reading data from the port in Perl.

Note that you may need to change the read_interval, read_char_time, and/or read_const_time settings in case you experience timeout problems.

Is your serial monitor a hardware device or a software on the PC? If it's the former, then your problem is indeed strange and I'd suspect incorrect settings or flow control issues somewhere. If it's the latter, then there might still be some driver issues going on preventing you from seeing things in your serial port monitor.

Lastly, see if you can get your PC and your Perl script talking to a different piece of hardware, such as a second PC running HyperTerminal or TeraTerm or similar.