ok, I've read the two above replies, and figured out a bare-bones (no error checking) way to do it. This program launches subroutine "ABC" a total of 20 times, but is only allowed to launch 6 at a time.

#!/opt/perl/bin/perl


# This is the simple child-process subroutine.
sub ABC
{
        sleep(2);
        print "abc with counter = $counter.\n";
}


# This section spawns the initial 6 children.

for ($counter = 1; $counter <= 6; $counter++)
{
        $pid = fork();

        if ($pid) # parent
        {
        $child{$pid} = $counter;
        $procnum[$counter] = $pid;
        print "forked process $counter.\n";
        }

        else # child
        {
        ABC();
        exit $counter;
        }
}


# This section spawns additional children 7-20.

$diecount = 0;

while ($counter <= 20)
{
        $doneproc = wait();
        $doneval = $? >> 8;
        $pid = fork();

        if ($pid) # parent
        {
        $child{$pid} = $counter;
        print "child $doneval ($doneproc) exited, forking process $cou
+nter.\n";
        $counter++;
        $diecount++;
        }

        else # child
        {
        ABC();
        exit $counter;
        }
}


# This section waits for all children to die.

while ($diecount <= 20)
{
        wait();
        $diecount++;
}

print "exiting.\n";
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BazB added code tags.