76.pl

package Course;
use Moose;

has 'students' => (
    is => 'ro',
    isa => 'ArrayRef[Student]',
    default => sub { [] },
);

sub add_student {
    my $self = shift;
    my @new_students = @_;

    $self->meta->get_attribute('students')
        ->type_constraint->assert_valid(\@new_students);

    $_->learns_at( $self ) for @new_students;
    push $self->students, @new_students;
}

package Student;
use Moose;
has 'name', is => 'ro', required => 1;
has 'learns_at', is => 'rw', weak_ref => 1;

package main;
my $c = Course->new;

$c->add_student( Student->new( name => 'Mike') );
warn $c->dump;