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