This is why win32/GNNUmakefile adds -Wno-format after -Wall and -Wextra
And the Cpanel-JSON-XS-4.38 Makefile.PL then oh-so-thoughtfully re-introduces the issue by appending "-Wall -Wextra -W" ....
I'll submit a PR for that Makefile.PL, along with a PR to XS.xs that allows Cpanel::JSON::XS to build on Windows quadmath perls (and also silences some other warnings).
Thanks tonyc - I remember that issue, now.
At one stage yesterday it crossed my mind that I don't usually encounter [-Wformat=] warnings ... but I still failed to make the connection :-(