  The most widely studied formation mechanism of a primordial black hole is
collapse of large-amplitude perturbation on small scales generated in
single-field inflation. In this Letter, we calculate one-loop correction to the
large-scale power spectrum in a model with sharp transition of the second
slow-roll parameter. We find that models producing an appreciable amount of
primordial black holes induce nonperturbative coupling on a large scale probed
by cosmic microwave background radiation. Our result implies that a small-scale
power spectrum can be constrained by large-scale cosmological observations.
