  We investigate the nuclear shadowing mechanism in the context of perturbative
QCD and the Glauber multiple scattering model. Using recent HERA data on
nucleon structure function at small $x$, we put stringent constrains on the
nucleon gluon density in the double-logarithm approximation. We suggest that
the scaling violation of the nucleon structure function in the region of small
$x$ and semihard scale $Q^2$ can be reliably described by perturbative QCD
which is a central key to the understanding of the scale dependence of the
nuclear shadowing effect. Our results indicate that while the shadowing of the
quark density arises from an interplay between the ``soft'' and semihard QCD
processes, the gluon shadowing is largely driven by a perturbative shadowing
mechanism. We demonstrate that the gluon shadowing is a robust phenomenon at
large $Q^2$ and can be unambiguously predicted by perturbative QCD.
