You are creating a 3 thread process. The simplest way to do what you've described would be:

use strict;
use warnings;
use threads;

async{
    open my $In, ...;
    open my $out, ...;
    while( <$in> ) {
        print $out;
    }
}->join;

[download]

And that's it. Of course it's not very useful as is, but then neither is what you posted.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re: Two threaded program by BrowserUk
in thread Two threaded program by Anonymous Monk

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