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();