That's a complex entity, rather beyond the scope of a "quick example". There are also some ambiguities in your description.

If processing is already done, they simply receive the output. If processing is not done, yet, they see the same (following) output of the process.

What if a read request is received for a process that has already started and produced half it's output? Does the new listener get everything that has already been sent to the file and any other listeners, or just that output the process produces after the request is received?

Anyway, here's how to use a thread to spawn a command and 'tee' it's output to a file, and retain a copy in memmory for your main thread to process at it convenience whilst also getting on with anything else it needs to do (like communicating with clients and monitoring queues):

#! perl -slw
use strict;
use threads qw[ yield async ];
use threads::shared;

my( $cmd, $file ) = @ARGV;
my $done : shared = 0;
my @lines : shared;

async {
    my $pid = open my $CMD, "$cmd |" or die "$cmd : $!";
    open my $fh, '>', $file or die "$file : $!";
    while( <$CMD> ) {
        chomp;
        print $fh $_;         ## output to the file
        push @lines, $_;    ## and push it to a shared array
    }
    $done = 1;
}->detach;

my $n = 0;
while( !$done ) {
    if( @lines ) {            ## lines to be processed
        print pop @lines;   ## process them
    }
    else {
        ## Else nothing to do but wait.
        yield;
    }
}
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Whether that would work in conjunction with POE I'm not sure.

Personally, I would write the whole thing using threads, but that's rather more than your OP asked for, and would take a fair amount of effort.

> That's a complex entity
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I agree

> some ambiguities in your description.
  (...)
> What if a read request is received for 
> a process that has already started and 
> produced half it's output?
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Of cause, the user/ node who attached to a process which has not terminated, yet, after the process has already produced some output, the user first received the old output and next recieves the on-going output as it is produced by the continuing process.

Thank you very much for your Thread example. I am excited about its cooperation with POE on win32.

Two clarifications just in case:

Thank you very much for your Thread example

• Please note: The example uses threads not Thread!

  They are different and incompatible modules. The former requires 5.8.x, preferably 5.8.4 or later.

  The latter was for versions earlier that 5.8.0, is deprecated, and will cease to function in 5.10.x.

  I am excited about its cooperation with POE on win32.

  Note: I said that I am not sure whether threads and POE will work together, they may not. I am not sure if anyone has tried this combination.

  ---

  Examine what is said, not who speaks.

  Silence betokens consent.

  Love the truth but pardon error.