I'm no Ruby expert, but I think you're misinterpreting the example in the book. @@population is a "class attribute", not an object attribute. In other words, it's a property of the "Person" class; not a property of each Person.

Using Moose you could model it along these lines:

package Person {
  use Moose;
  use MooseX::ClassAttribute;

  has name => (is => 'ro', isa => 'Str');

  class_has population => (is => 'ro', isa => 'ArrayRef');
}

my $bob = Person->new(name => "Robert");
push @{ Person->population }, $bob;
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Though there are many reasons to avoid class attributes. (Basically think of all the good reasons to avoid global variables, and then s/global variables/class attributes/gi.)