Previously defined covariant and gauge-invariant perturbation variable
s, representing, e.g., the fractional spatial energy density gradient
on hypersurfaces of constant expansion, are used to simplify the linea
r perturbation analysis of a classical scalar field. With the help of
conserved quantities on large scales we establish an exact first-order
relation between comoving fluid energy density perturbations at ''ree
ntry'' into the horizon and corresponding scalar field energy density
perturbations at the first Hubble scale crossing during an early-de Si
tter phase of a standard inflationary scenario.