Pre-big bang cosmology predicts tiny first-order dilaton and metric perturbations at very large scales. Here we discuss the possibility that other -- more copiously generated -- perturbations may act, at second order, as scalar seeds of large-scale structure and CMB anisotropies. We study, in particular, the cases of electromagnetic and axionic seeds. We compute the stochastic fluctuations of their energy-momentum tensor and determine the resulting contributions to the multipole expansion of the temperature anisotropy. In the axion case it is possible to obtain a flat or slightly tilted blue spectrum that fits present data consistently, both for massless and for massive (but very light) axions.