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use base Vehicle;
@ISA = 'Vehicle';

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

...
    my ($self, $option) = @_;
    return defined $ro{$option} ? 1 : 0;
}

sub _attributes
{
    return
...
    my ($self, $attribute) = @_;
    return exists $self->_attributes()->{ $attribute };
}