b has a %^H and c doesn't. The extra time spent is clearly spent accessing (copying?) this hash.

Why doesn't c have a %^H?

Some hints are found in $^H and some are in %^H.

I think what's happening is that use 5.010; avoids creating %^H just to set flags that are on by default (such as indirect).

use feature ":5:10";, on the other hand, clearly does create this hash.

There's a problem with this answer. say and a couple other flags which aren't set by default appear to be in %^H. Maybe they are found in both $^H and %^H?