Note that there need not be a problem of sync between the threads. They do not write to common fields. as I mentioned, I actually use this in an object so I pass $self, but this example also illustrates the problem:

##! perl -slw
use 5.010;
use strict;
use threads;
use Thread::Queue qw( );

sub sub1 { say"sub1 starts"; sleep 1 for 1..2; say"sub1 ends" }
sub sub2 { my $hashref = shift; $hashref->{KEY}=1; exists($hashref->{K
+EY}) ? say "KEY exists" : say "KEY does NOT exist";
    say"sub2 starts"; sleep 1 for 1..2; say"sub2 ends" }
sub sub3 { say"sub3 starts"; sleep 1 for 1..5; say"sub3 ends" }
sub sub4 { say"sub4 starts"; sleep 1 for 1..5; say"sub4 ends" }
sub sub5 { say"sub5 starts"; sleep 1 for 1..5; say"sub5 ends" }
sub sub6 { say"sub6 starts"; sleep 1 for 1..5; say"sub6 ends" }
sub sub7 { say"sub7 starts"; sleep 1 for 1..2; say"sub7 ends" }
sub sub8 { say"sub8 starts"; sleep 1 for 1..2; say"sub8 ends" }

my $hashref = {};

my $q = Thread::Queue->new();
my $t1 = async { sub1(); $q->enqueue(1); };
my $t2 = async { sub2($hashref); $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) {
      $t2->join();
      exists($hashref->{KEY}) ? say "KEY exists" : say "KEY does NOT e
+xist";
      push @threads, async { sub5() }, async { sub6() };
      --$pending;
   }

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

$_->join() for @threads;
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See threads::shared

thanks, I just read it, but I remained puzzled. I remind you that all these subroutines (sub_#) object's subroutines that are called from another subroutine (sub run) of the same object. So the calling subroutine gets $self and should pass $self to each of the subs. Should I share $self at the beginning of the calling subroutine (sub run)? This does not seem to make sense since the doc says that sharing deletes the content.