There are some really good explanations in the blog post link I have provided. A simple answer is that if the attribute value is a reference, you can easily use the reference to change the object. Consider a Person object that has some friends, with friends being an array reference. You can make the friends attribute 'ro', but you gan still read the reference and use it to change the object, like this:
push @{ $john->friends }, $new_friend;
That is why I'm asking for a shortcut to writing is => 'ro', and not for a way to make the object immutable. I don't think that making the object truly immutable is a sensible goal. Making all the attributes readonly (in the way Moo understands it), on the other hand, can be useful.
