package ClassOne;

sub new {
   my ($class, %args) = @_;
#...
  my $data = $class->load_a_bunch_of_data($arg{one});
  return bless $data, $class;
}

use ClassThreeList;
sub load_a_bunch_of_data {
  my ($class, $arg1) = @_;
  # load a bunch of data that only ClassOne cares about
  # ...
  # load a list of data that both ClassOne 
  # and ClassTwo care about.
  my $item_list = ClassThreeList->load_where(objectlistid => $arg1);
  foreach my $item (@$item_list) {
    ### BEGIN CODE THAT I WANT TO SHARE ###
    $item->load_attr1_list;
    $item->load_attr2_list;
    $item->calculate_some_attribute.
    $item->etc;
    ### END CODE THAT I WANT TO SHARE ###
  }
}

# ClassTwo does not exist yet.