open(MFOUT,">mfout.txt");
while (<STDIN>)
{
 print MFOUT $_;
 print  $_;
 <SNIP>
 ... stuff that builds up commands in @commands...
 ... eg 'perl anotherscript.pl < input' 
 </SNIP>
}

my %running = ();
my $max_jobs = 200;
my $job_count = 0;

while ( @commands || %running )
{
  if ( @commands && ($job_count < $max_jobs))
  {
    my $command = shift(@commands);
    my $pid;
    if ($pid = fork)
    {
         print "Parent Pid $$\n";
         $running{$pid} = $command;
         ++$job_count;
    }
    else
    {
         die "cannot fork: $!" unless defined $pid;
         print "Child Pid $$ $command\n";
         my $rc = system $command;
         exit($rc>>8);
    }
  }
  else
  {
   my $child_pid = wait();
   if (! exists $running{$child_pid})
   {
     warn "Reaped unkown process id $child_pid!!";
   }
   elsif ($?)
   {
     my $rc = $? >> 8;
     warn "Process '$child_pid:$running{$child_pid}' errored with return code '$
rc'";
   }  
   print "command '$running{$child_pid}' completed\n";
   delete $running{$child_pid};
   --$job_count;
  }
}