I find: ~0 > 4294967295 to work and be simple/fast/understandable. Easier to follow at a glance than your pack, no time spent loading Config, no log() needed. If you use it a lot, make it a constant, e.g.

use constant MAXBITS => (~0 > 4294967295) ? 64 : 32;

A downside is that it assumes a 32-/64-bit world so isn't ready for 128-bit UVs if they every come. If you're already using the Config module or think it's more clear and don't care about the small load time, then I'd use that ($Config{uvsize}).

A warning on using ~0: don't use it in a place where bigint, bignum, or integer may be loaded, because it will be -1. This shouldn't be an issue inside your module, but something to be aware of.