Thanks for your explanation. Was able to work around this by opening files with the FILE_SHARE_DELETE permission as you using Win32API::File and it does seem to do the trick. Replaced the thread code with:

    my $tid = threads->tid;
    my $fn = "file" . $tid;

    my $winFileHandle = createFile( $fn, "w", "d" ) or die "$!/$^E";
    OsFHandleOpen(*OUTPUT, $winFileHandle, "w") or die "$!/$^E";
    
    my $pid = open(my $pipe, "-|", "perl test.pl");
    
    print OUTPUT $_ while(<$pipe>);

    close(OUTPUT);
    close($pipe);
    CloseHandle($winFileHandle);
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I do however have another question, well more of a clarification. So in my actual usage, I am using a threadpool and a Thread::Queue and not starting/joining threads after one another. But, I'm just making sure I understand correctly, this does not matter because if I have for example:

1. thread 1 open file1
2. thread 2 start process A while file1 is still open
3. thread 1 closes file1

process A will still have a filehandle for file1 even though thread2 did absolutely nothing with the file because the spawned process inherits ALL of the the file handles currently in use by the main perl process at the time of opening the pipe? Sorry if this comes out more as just paraphrasing what you said, just wanting to make sure I understand fully :)

Thanks for the help!