There is another, similar error at line 123 of the module: $self->{cw} = 0;    # Reset

Which should be ${ $self->{cw} } = 0;    # Reset

With that also fixed, the code does appear to run, and the shared file does receive output from both threads.

It still exits with 2 threads still running, and there does not appear to be any mechanism code for dealing with that.

It is intriguing code. Whomever wrote it knows a lot more about the cond_* calls than I do--but then I have made it my habit to avoid them.