You could do it the easy way. 

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

my $Qrv = new Thread::Queue;

my @work = qw[ C: D: P: Q: T: U: V: W: Z: ];

for ( 1 .. 2 ) {
    my $cmd = join ' ', 'dir /s', shift( @work ),  '>nul';
    async( sub{ $Qrv->enqueue( "$_: " . system( $cmd ) ); } )->detach;
}

while( @work ) {
    my( $threadNo, $rv ) = split ': ', $Qrv->dequeue();
    my $cmd = join ' ', 'dir /s', shift( @work ),  '>nul';
    async( sub{ $Qrv->enqueue( "$threadNo: " . system( $cmd ) ); } )->
+detach;
}

sleep 2;

[download]

A slightly more verbose version shows the workings 

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

my $Qrv = new Thread::Queue;

my @work : shared = qw[ C: D: P: Q: T: U: V: W: Z: ];

for (1..2) {
    my $work = shift( @work );
    print "Starting '$work'";
    my $cmd = "dir /s $work >nul";
    my $thread = async{ $Qrv->enqueue( "$_: " . system( $cmd ) ); };
    $thread->detach();
}

while( @work ) {
    print "Work left: @work";
    my( $threadNo, $rv ) = split ': ', $Qrv->dequeue();
    printf "Thread: $threadNo returned %d\n", $rv>>8;
    my $work = shift( @work );
    print "Starting '$work'";
    my $cmd = "dir /s $work >nul";
    my $thread = async{ $Qrv->enqueue( "$threadNo: " . system( $cmd ) 
+); };
    $thread->detach();
}

# Ought to yield here until no threads are left running
# but the API doesn't provide any way to find out how many detached th
+reads
# are left running.
# Equally, there is no way to join() the next thread to finish.
sleep 2;

__END__
P:\test>357420
Starting 'C:'
Starting 'D:'
Work left: P: Q: T: U: V: W: Z:
Thread: 1 returned 0
Starting 'P:'
Work left: Q: T: U: V: W: Z:
Thread: 1 returned 0
Starting 'Q:'
Work left: T: U: V: W: Z:
Thread: 1 returned 0
Starting 'T:'
Work left: U: V: W: Z:
Thread: 1 returned 0
Starting 'U:'
Work left: V: W: Z:
Thread: 1 returned 0
Starting 'V:'
Work left: W: Z:
Thread: 1 returned 0
Starting 'W:'
Work left: Z:
Thread: 2 returned 0
Starting 'Z:'

[download]
Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail

In reply to Re: Keep Two Processes Going by BrowserUk
in thread Keep Two Processes Going by blax

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