This is in response to Matt Stauffer's blog post Form array validation in Laravel 5.2.
At the end Matt explains that you can apply validation rules to a specific array index, he also says that he is "Not sure why you would do this, but, it's possible".
One scenario I have thought of is when you need to require at least one value for a multiple value field, for example a user can have many email addresses.
$this->validate($request->all(), [
'email_addresses.1' => 'required|email',
'email_addresses.2' => 'email',