I think your basic logic error is here:

my $pm = Parallel::ForkManager->new(50);  # 50 forks
my $i = 0;
while(i<50){  # start 50 forks
  $pm->start and next;
  processStream($stream);
  $i++;
  $pm->finish; # do the exit in the child process
}
$pm->wait_all_children();  # and then do nothing
[download]

I'd amend your code to this (adjusting the fork-amount to something sane such as 4):

# max 4 processes simultaneously
my $pm = Parallel::ForkManager->new(4);
while( .. there are rows to fetch ..){
  $pm->start and next;
  processStream($stream);
  $pm->finish; # do the exit in the child process
}
$pm->wait_all_children();
[download]

The amount of rows to fetch could be 50 or whatever number that you want. Parallel::ForkManager will take care of the queueing and will keep four forks alive as long as there are jobs in the queue.