You should create your variables shared. Sharing objects doesn't really make sense unless the object is specifically designed to transport data between threads. Either special purpose objects, or don't use objects at all to exchange data.

Oh... I don't get it. All of these subs are within the same object. Each of them does some changes to the object state (e.g. add stuff to $self).

Can't I make $self shared? I don't really understand what is the alternative?

Here is a simple example (following the previous one):

obj.pm:

package obj;

use 5.010;
use strict;
use threads;
use Thread::Queue qw( );

sub new {
    my ( $class, %args ) = @_;
    my $self = bless( {}, $class );
    say "creating new obj";
    return $self;
}

sub sub1 { say "sub1 starts"; sleep 1 for 1 .. 2; say "sub1 ends" }

sub sub2 {
    my $self = shift;
    $self->{KEY} = 1;
    exists( $self->{KEY} ) ? say "KEY exists" : say "KEY does NOT exis
+t";
    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" }

sub run {
    my $self    = shift;
    my $q       = Thread::Queue->new();
    my $t1      = async { sub1(); $q->enqueue(1); };
    my $t2      = async { sub2($self); $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( $self->{KEY} )
              ? say "KEY exists"
              : say "KEY does NOT exist";
            push @threads, async { sub5() }, async { sub6() };
            --$pending;
        }

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

    $_->join() for @threads;
}

1;
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test.pl:

use strict;
use warnings;

use obj;

my $o = obj->new();
$o->run();
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