Since posting I've found that there are, in fact, two loadings of Scalar::Util in the strace dump. The first one finds my version as desired. The second does not! Since this was an strace -fF on make, not perl, it may very well be that a second interpreter (whether another installation or another instance of the same one) is being invoked. But the question is now: under what circumstances is the environment override (which prepends to @INC, by design of course) ignored? Or is there someone who is weeding my @INC?

Hmmm. Maybe taint mode does this! *dives in again*