use strict;
use warnings;

use threads;

use Thread::Queue qw( );

my $q = Thread::Queue->new();
my $t1 = async { sub1(); $q->enqueue(1); };
my $t2 = async { sub2(); $q->enqueue(2); };

my $pending = 2;
my @threads;
for (;;) {
   my $id = $q->dequeue();
   if ($id == 1) {
      $t1->join();
      push @threads, async { sub3() }, async { sub4() };
      --$pending;
   }
   elsif ($id == 2) {
      $t2->join();
      push @threads, async { sub5() }, async { sub6() };
      --$pending;
   }

   if (!$pending) {
      push @threads, async { sub7() }, async { sub8() };
      last;
   }
}

$_->join() for @threads;