use base Vehicle;
@ISA = 'Vehicle';
[download]

Pick one or the other, not both. I prefer base.

Use unless.

my %valid = map { $_ => 1 } qw( Wheels Doors Color Passengers );
my %ro    = map { $_ => 1 } qw( Wheels );

sub _read_only {
    my ($self, $option) = @_;
    return defined $ro{$option} ? 1 : 0;
}
[download]

If you elided the lexicals in favor of class methods, you wouldn't have to repeat this code in both subclasses:

sub _attributes
{
    return
    {
        map { $_ => 1 } qw( Wheels Doors Color Passengers )
    };
}

sub _valid
{
    my ($self, $attribute) = @_;
    return exists $self->_attributes()->{ $attribute };
}
[download]

Finally, aside from inheriting a constructor and an AUTOLOAD, I don't see any reason why you need to inherit here. You'd probably be better off using a module that autogenerated your accessors. It's not wrong, but I don't see any real benefits to this approach.

I did it this way when I was first learning, though. It won't cause you any real trouble; it's just a bit more complication that doesn't add very much. ~shrug~