Since when has VS-compatibility been such a high priority for Perl development?

Given that there are vast tracts of Perl source that need to be conditionally compiled for different OSs, the requirement that some gcc-enabled C99 syntax might need additional effort for VS is neither here nor there.

That said, as of this month, even gcc doesn't do all of C99. But that's no good reason to restrict the use of features that have been common extensions to virtually every C compiler since long before C99 was ratified.