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.