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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