A few debugging ideas:

1. $PortObj->error_msg(1);
   $PortObj->user_msg(1);
   [download]

2. Check for error messages from the module, e.g. ... or die "Error: $^E";

3. my $count_out = $PortObj->write($output_string);
   warn "write failed\n" unless $count_out);
   warn "write incomplete\n" if $count_out != length($output_string);
   [download]

4. Check if you need to set up any flow control, if not, use $PortObj->handshake("none");
5. Make sure you're setting the CTS, DTR, etc. lines appropriately, if they are connected and being used in hardware.