sub new
{
        my $class = shift;
        my %field = (
                foo    => 1,
                bar    => 1,
                baz    => 1,
                class  => $class,
        );

        bless sub {
                my $name = shift;

                my ($package, $filename, $line) = caller;
                die "Attempt to access private class data " .
                        "for $field{class} at $filename line $line\n"
                        unless UNIVERSAL::isa( 
                            $package => __PACKAGE__ );

                die "No such field '$name' at $filename line $line\n"
                        unless exists $field{$name};
                die "You can't change the class name " . 
                        "at $filename line $line\n"
                        if $name eq 'class';

                $field{$name} = shift if @_;
                $field{$name};
        } => $class;
}