package Rider;
sub new {
 my($pck, $name)=@_;

 return bless { name => $name }, $pck;
}

package Horse;
sub new {
 my($pck, $horse_name, $rider_name)=@_;

 return bless { 
               name => $horse_name,
               rider => Rider->new($rider_name),
              }, $pck;
}

You can create instances with Horse->new("Mr Ed", "John Wayne"). The created class will have a "rider"-attribute which is an instance (= blessed reference) of the rider-class.

So yes you simply store the object-references in your underlying data-structure.

And by the way: The reason I bless not directly into "Rider" and "Horse" namespaces but rather use the package-name that the constructor gets passed when called via e.g. Rider->new (you should NOT call it as Rider::new) is so that I can re-use the constructor in a derived class - read Damian's book if this is unclear.

hth