I hope I am not seen as trying to be or indeed being pedantic but the documentation of the JSON::Parse version I was using (0.40) (and before my question, it was the latest at CPAN) stated thus:

warn_only
...
This method was added in version 0.38.
[download]

(along similar lines the comment by stevieb earlier)

Now, after stevieb had placed a ticket, the latest version is 0.41, apparently the issue has been fixed and the relevant documentation states thus:

This method was documented in version 0.38, but only implemented in ve
+rsion 0.41.
[download]

MANY THANKS to the author of JSON::Parse (Ben Bullock) for providing the module and fixing this minor but irritating issue in speedy fashion.

bliako