I read the documentation for detach and it scared me a little bit :)

So avoid detach and use a couple more joins:

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

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

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

$_->join for $t1, $t2;

my $t3 = async{ sub7() };
sub8();

$t3->join;
print "main ends";
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