In the case of the IV, the greyed-out bits represent memory that doesn't actually exist. The IV value is actually stored in the svu_pv slot in the head, and the sv_any pointer in the head actually points back to itself (with a slight offset), so that it appears to be pointing to a struct whose fourth field is an xiv_iv. In this way only a head and not a body needs allocating, but all the perl functions which expect to find it as the fourth field of the body still work.

Dave.