Actually when you use void in the XS sub declaration it does not prevent you using OUTPUT (although this is now deprecated) SV* is now preferred to void..... There are even examples of doing it that way in perlxstut.

Older versions of this document recommended to use void return value i
+n such cases. It was discovered that this could lead to segfaults in 
+cases when XSUB was truly void. This practice is now deprecated, and 
+may be not supported at some future version. Use the return value SV 
+* in such cases. (Currently xsubpp contains some heuristic code which
+ tries to disambiguate between ``truly-void'' and ``old-practice-decl
+ared-as-void'' functions. Hence your code is at mercy of this heurist
+ics unless you use SV * as return value.)
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