You are mostly right, in the end he does want to run 1000 jobs, but only have 100 at a time max and in blocks of 10.

When calling P::FM you declare how many forks you want to run. In the main method:

my $max_children   = 100;
my $job_size       = 10;

my $main_pfm = P::FM->new( $max_children/$job_size );
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So that is 100/10 which is 10 job blocks at the same time. Since the job size is 10, then that is 100 jobs at the same time.

my $total_children = 1000;
my $job_size       = 10;

foreach ( 1..$total_children/$job_size ) {
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So then we loop through our data (F::PM blocks if there is no avalible slots) 1000/10 is 100. So the main method loops through 100 job blocks while only running 10 blocks at a time.

I have read a limitation of P:FM that you cannot define new P:FM until all Child processes from the previous are finished. This limitation is mentioned in CPAN for P::FM. So can any one give another suggestions?

Almost:

You are free to use a new copy of Parallel::ForkManager in the child processes, although I don't think it makes sense.

What you saw was:

If you want to use more than one copies of the Parallel::ForkManager, then you have to make sure that all children processes are terminated, before you use the second object in the main program.

However, I would test it to double check. Using the run_on_finish callback would work. Whether it would be more clear or easier to read is up to the user.

You can't define another P::FM object within the same process (until all child processes from the previous are terminated...), but you can define a new object in a child process.