The haphazard syntax of Readonly (not to mention its dueling APIs) trips up more people and generates more email than any of you guys can imagine... An extra warning that perl itself no longer throws may help clear up some of the 'weird' things people run into so I'll take this patch.
Generating a new fatal error though may 'break' a lot of code. Then again if they were using this incorrect syntax their code was already broken, right? ((sigh)) I'll push it to PAUSE as a dev build for a few weeks. At the end of February (or March), I'll make it stable along with the mutable clone stuff I wrote almost two years ago.
Edit: Well! It's a minor change but I like giving credit where it's due so please feel free to submit a pull request here.