my $foo = Foo->new(one => 1);
is($foo->one, 1, 'ro reads');
-ok(!eval { $foo->one(-3); 1 }, 'ro dies on write attempt');
+ok(exception { $foo->one(-3) }, 'ro dies on write attempt');
is($foo->one, 1, 'ro does not write');
is($foo->two, undef, 'rw reads');