  We investigate the pion scalar form factor in the Meiman-Okubo framework,
implementing the phase below the inelastic $K\bar K$ threshold, where it is
known from the $\pi\pi$ scalar isoscalar phase shift $\delta_0^0$ by Watson
theorem. State-of-the-art knowledge of the perturbative QCD expansion of the
scalar correlator and the phase shift $\delta_0^0$ is used as input. No
assumptions about the phase above the inelastic threshold or the possible zeros
of the form factor in the complex plane are necessary. We obtain a
model-independent constraint relating the sum of the light quark masses to the
slope and the curvature of the pion scalar form factor at the origin. The
recent lattice results for the light quark masses and the pion scalar radius
are found to satisfy this constraint. We obtain also a strong correlation
between the pion scalar radius and the curvature of the form factor, with
rather high values predicted for the curvature.
