  Standard power-behaved contributions in QCD arising from non-perturbative
effects at low scale can be described, as shown by Dokshitzer, Marchesini and
Webber, with the notion of an infrared regular effective coupling. In their
approach, a non-perturbative contribution to the coupling, essentially
restricted to low scales, parametrizes the non-perturbative power corrections.
I argue that their framework naturally allows for another type of power
contributions, arising from short distances (hence unrelated to renormalons and
the operator product expansion) which appear in the process of removing the
Landau singularity present in perturbation theory. A natural definition of an
infrared finite perturbative coupling is suggested within the dispersive
method. Implications for the tau hadronic width, where $O(1/Q^2)$ contributions
can be generated, are pointed out.
