It may not be a good idiom, but usually I only check wantarray for a true value, and just return a scalar value in either scalar or void context and just let Perl throw it away if unused. I think that's probably reasonable for anything for which the typical case is to return one or more values and I don't see much point in adding the void case just to be pedantic.

For fork, the last time I did it, I used a three-part if/elsif/else for clarity.