I had an earlier post removed, presumably because I subconsciously knew this answer, but hadn't explicitly expressed it. (I had mumbled something along the lines of that this is the type of behaviour I would naturally expect).

I suppose anyone with a good grounding in C would expect this answer.. it would only be a surprise to someone who hadn't used atof or atoi before that the '2x' + '3y' question would give a surprising answer.

So, indeed, given that the Camel book explicitly states that atof is used, and given that atof is documented, it follows that this is indeed a documented feature of Perl. I will mod up the post to which this is a reply to.