P::FM forks a child for each "task". If you provide a limit (e.g. the 2), it waits for children to finish before forking if forking would bring the number of children over the limit.

use v5.14;
use warnings;

use Parallel::ForkManager qw( );
use Time::HiRes           qw( sleep );

my $pm = Parallel::ForkManager->new ( 2 );

for ( 1 .. 9 ) {
   $pm->start and next;
   say "[$_] start";
   sleep( $_ == 1 ? 4 : 0.3 );
   say "[$_] done";
   $pm->finish;
}

$pm->wait_all_children();
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Annotated output:

                   1   2   3   4   5   6   7   8   9
           -----  --- --- --- --- --- --- --- --- ---
[1] start  1      XXX
[2] start  1 & 2  XXX XXX
[2] done   1      XXX 
[3] start  1 & 3  XXX     XXX
[3] done   1      XXX 
[4] start  1 & 4  XXX         XXX
[4] done   1      XXX 
[5] start  1 & 5  XXX             XXX
[5] done   1      XXX 
[1] done
[6] start  6                          XXX
[7] start  6 & 7                      XXX XXX
[6] done   7                              XXX
[7] done
[8] start  8                                  XXX
[9] start  8 & 9                              XXX XXX
[8] done   9                                      XXX
[9] done
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