It is easy enough to set a define in the Makefile.PL if use64bitint is set

Just be a little cautious with $Config{use64bitint}. It doesn't always tell you what you want/need to know. On 32-bit systems where perl is built with -Duse64bitint, the 'long' and 'int' sizes can (and generally do, I believe) remain at 4 bytes.

I think I've also seen perls built with -Dusemorebits (the equivalent of building with -Duse64bitint && -Duselongdouble) that have neither use64bitint nor uselongdouble defined.

And finally, it would be possible to have 64-bit longs and ints in play without having built with use64bitint support (ie when 64 bits is the size of the long/int on the particular compiler being used).

There are probably other aspects to consider as well. (See the INSTALL file that ships with the perl source for a more authoritative account.)

Cheers,
Rob