Module::Build's XS support is still in alpha phase (according to its POD), and still somewhat buggy (I've submitted bug reports for all of the items below).

For XS support to work, ExtUtils::ParseXS must be installed. Module::Build doesn't check for it, so you must add it to 'build_requires' in Build.PL.

Module::Build doesn't generate VERSION and XS_VERSION for compiling the C code generated from the XS code. You need to add them to 'extra_compiler_flag' in Build.PL.

Your Build.PL file should check for have_c_compiler() if you plan to upload it to CPAN.