  Demanding the analyticity of hadronic observables (calculated in terms of
power series of the running coupling) as a {\it whole}, we show that they are
free of the Landau singularity. Employing resummation and dispersion-relation
techniques, we compute in a unifying way power corrections to two different
hard-scattering functions in perturbative QCD: the electromagnetic pion form
factor to leading order and the inclusive cross section of the Drell-Yan
process. In the second case, the leading nonperturbative power correction in
$b\Lambda_{\rm QCD}$ gives rise to a Sudakov-like exponential factor in the
impact parameter space which provides enhancement rather than suppression.
