my $task = $rs_task->create({
   name => 'Second task',
   subtask => {
       title => 'First subtask',
       ....
 
   }
});

##</code><code>##

my $task = $rs_task->create({
   name => 'Second task',
   subtask => $subtask_rs->new({
       title => 'First subtask',
       ....
 
   })
});