Not to add to your choices, but whenever I am faced with this I use Hash::Util's lock_keys method as follows....

package Dog;
use Hash::Util qw(lock_keys);
use Data::Dumper;

sub new
  {
    my $pkg = shift;
    my $ref = {};
    bless $ref, $pkg;
    lock_keys(%$ref,qw(intelligence));
    $ref->{intelligence} = shift;
    $ref;
  }

sub is_stupid {
    my $self = shift;
    return $self->{inteligence} < 3;
}


my $lassie = Dog->new(10);
print Dumper($lassie);
print "Lassie is ", $lassie->is_stupid ? "dumb" : "smart";
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In the case of subclassing this, it should be easy enough to call unlock_keys, do what you need, and then lock it again.

This novice wasn't aware of the other methods you mention for achieving this, and would be interested in hearing what other monks might say of the methods described here.

Of course this only gives one runtime checks, but those are sufficient for my purposes.