Ought there be a %Config setting for the assembler ?

Hmm, perl doesn't use it, so I guess not :/ maybe :)

If you look at Win32-API-0.77\Makefile.PL you'll see it uses $(AS) but it doesn't define it , which probably means it simply defaults to 'as'

You can always arrange for  perl Makefile.PL AS=x86_64-w64-mingw32-as.exe

dmake should also pick it up from %ENV via set AS=x86_64-w64-mingw32-as.exe or via sitecustomize $ENV{AS}='x86_64-w64-mingw32-as.exe';

Or give it to Makefile.PL via $ENV{PERL_MM_OPT} https://metacpan.org/pod/ExtUtils::MakeMaker#PERL_MM_OPT

So no Makefile hacking required, set some %ENV vars as usual :)

I guess whomever built your toolchain chose x86_64-w64-mingw32-as.exe so they can dump all the .exe in the same directory :) choices complicate choices :)