Seeing your having trouble with the forking, you could try this and see how it fairs. I've rather over-commented the code. HTH.

#! perl -slw
use strict;
use threads qw[ yield ];
use Thread::Queue;

## The command to run (DIR for testing.
my $COMMAND = 'dir /s';

## FIFOs in and out
my $Qwork = new Thread::Queue;
my $Qresults = new Thread::Queue;

## Track our workers
my $workers :shared = 0;

sub worker {
    $workers++; ## Count 'em in
    threads->self->detach; ## No return
    
    ## Wait until we got something to do
    yield until $Qwork->pending;
    
    ## Whilst there is work
    while ( $Qwork->pending ) {
        
        ## Grab some
        my $task = $Qwork->dequeue;
        
        ## do it and get the output
        open my $in, "$COMMAND $task 2>&1 |" 
            ## Maybe queue it back rather than yelling
            or warn "command $task failed: $!";
        
        ## Grab the output and post it back to the main thread
        ## Prefix with the work item if it must be segregated
        $Qresults->enqueue( $_ ) while <$in>;        
    }
    $workers--; ## and count 'em out again
}

## Start a pool of workers
threads->new( \&worker ) for 1 .. 3;

## Give 'em something to do
$Qwork->enqueue( $_ ) for qw[ D: P: Q: T: U: V: W: Z: ]; ## Drive to d
+ir

## wait for something to do
yield until $Qresults->pending;

## Workers running
while( $workers ) {
    ## get a result if available
    my $result = $Qresults->dequeue if $Qresults->pending;

    ## Do something with $result
    printf $result if defined $result;
    
    ## Give up the timeslice if workers are running
    ## but we have no results yet
    yield while $workers and not $Qresults->pending;
}

## All done.
[download]