Sorry, you're right. How did we miss that. How's this?

#! perl -slw
use 5.010;
use strict;
use threads;

=commant
sub_1 can run immediately
sub_2 can run immediately
sub_3 can run only after sub_1 finishes
sub_4 can run only after sub_1 finishes
sub_5 can run only after sub_2 finishes
sub_6 can run only after sub_2 finishes
sub_7 can run only after both sub_1 and sub_2 finish
sub_8 can run only after both sub_1 and sub_2 finish
=cut

sub sub1 { say"sub1 starts"; say("sub1:$_"),sleep 1 for 1..3; say"sub1
+ ends" }
sub sub2 { say"sub2 starts"; say("sub2:$_"),sleep 1 for 1..3; say"sub2
+ ends" }
sub sub3 { say"sub3 starts"; say("sub3:$_"),sleep 1 for 1..3; say"sub3
+ ends" }
sub sub4 { say"sub4 starts"; say("sub4:$_"),sleep 1 for 1..3; say"sub4
+ ends" }
sub sub5 { say"sub5 starts"; say("sub5:$_"),sleep 1 for 1..3; say"sub5
+ ends" }
sub sub6 { say"sub6 starts"; say("sub6:$_"),sleep 1 for 1..3; say"sub6
+ ends" }
sub sub7 { say"sub7 starts"; say("sub7:$_"),sleep 1 for 1..3; say"sub7
+ ends" }
sub sub8 { say"sub8 starts"; say("sub8:$_"),sleep 1 for 1..3; say"sub8
+ ends" }

my $t1 = async{
    sub1();
    return async {
        my $t = async {
            sub3();
        };
        sub4();
    $t->join;
    };
};

my $t2 = async{
    sub2();
    return async {
        my $t = async{
            sub5();
        };
        sub6();
        $t->join
    };
};

my @kids;
push @kids, $_->join for $t1, $t2;

push @kids, async{ sub7() };
sub8();

$_->join for @kids;
print "main ends";

__END__
c:\test>854022.pl
sub1 starts
sub1:1
sub2 starts
sub2:1
sub1:2
sub2:2
sub1:3
sub2:3
sub1 ends
sub2 ends
sub4 starts
sub3 starts
sub3:1
sub4:1
sub6 starts
sub5 starts
sub6:1
sub5:1
sub8 starts
sub8:1
sub7 starts
sub7:1
sub3:2
sub4:2
sub5:2
sub6:2
sub8:2
sub7:2
sub3:3
sub4:3
sub5:3
sub6:3
sub8:3
sub7:3
sub3 ends
sub4 ends
sub5 ends
sub6 ends
sub8 ends
sub7 ends
main ends
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