in reply to Re^9: Module for 128-bit integer math?
in thread Module for 128-bit integer math?

Support for unsigned __int128 types was probably not yet implemented

A post to the mingw64 mailing list suggested that I create the __uint128 type myself (in the compiler's _mingw.h) with:
typedef unsigned int __uint128 __attribute__ ((__mode__ (TI)));
and that works fine.
Then, in Int128.xs, it was just a matter of changing:
typedef unsigned __int128 uint128_t; to typedef __uint128 uint128_t;
BrowserUk (or anyone else, of course), if you want a ppm package of that module that will work with the 64-bit builds of ActivePerl (both perl-5.10 and perl-5.12):
ppm install