is all it takes. Thanks.
Of course, in the meantime I'd figured out that -mms-bitfields would probably *not* be the cause of my problems, as the crash I get (on ActivePerl only) happens at cleanup - whereas the absence of -mms-bitfields will cause problems much earlier than that.
So now I just need to figure out why my mingw-built binaries work fine on mingw-built perl, but crash on exit when run on ActivePerl.