The use of semiconductor nanowires as new material building blocks for developing original devices is conditioned by the controllability of their growth. An important challenge is to form nanowires which include heterostructures of predictable dimensions. This objective requires a precise knowledge of the growth kinetics which appears much more complex for nanowires than for standard two-dimensional layers. Here, we present a method which provides detailed information on nanowire formation. The method is implemented with InP1-xAsx nanowires grown by Au-catalyzed molecular beam epitaxy. Controlled and periodic modulations of the incident vapor phase are generated. Due to these modulations, the nanowires show small and short oscillations of composition along their growth axis. These oscillations furnish a time scale which is recorded in the nanowire solid phase. The instantaneous growth rate and the total length of individual nanowires at any time of the growth are accessible. Moreover, the distribution of the oscillation lengths contains the nucleation statistics. This statistics is shown to be strongly sub-Poissonian, which indicates that some regulation mechanism operates. The rapid depletion of group V atoms in the catalyst drop which follows the growth of each ML could explain the self-regulation of nucleation events.