...I seem to recall that map was just rewritten to check for void
context and avoid the extra work if possible.

    That is correct. And it's in Perl 5.8.1!
    From the 5.8.1. perldelta:

        "map" in void context is no longer expensive. "map" is now
        context aware, and will not construct a list if called in
        void context. 

##</code><code>##

Larry Wall (<larry {at} wall.org>)
Re: grep/map in void context
perl.porters-gw

The argument against using an operator for other than its primary
purpose strikes me the same as the old argument that you shouldn't
have sex for other than procreational purposes.  Sometimes side
effects are more enjoyable than the originally intended effect.