All of perl requires a ANSI C compliant compiler.

CPAN != perl.

That means that there is no guarantee whatsoever that you can build modules written for C99 minimum can be used with any version of perl.

Irrelevant, as JSON::XS obviously can be build against perl. cpantesters only shows 1 fail, and almost 300 passes for its latest version, which is a pretty good record.

I did send a patch to make that source C89 compliant, but it was rejected as "the code is already compliant to the standard" (and then deleted).

Kudos for writing a patch. But it's the authors right to reject patches, for whatever reason. However, don't despair. The license of JSON-XS is "This module is licensed under the same terms as perl itself." Which means you can apply your patches, and distribute them. Feel free to put the modifications on CPAN and distribute them.