Sorry for creating any confusion by using two different compilers. It seems clear that the problem is that the "quadmath" library isn't loaded. The problem is not that the "quadmath" isn't found. It will be found, when requested. Alas, I don't know where in the build process I need to add that when Perl is built with the -Dusequadmath option, the "quadmath" library should be loaded. --- via perlbug: queue: perl5 status: open https://rt.perl.org/Ticket/Display.html?id=128627