  Mirror matter is a dark matter candidate. In this paper, we re-examine the
linear regime of density perturbation growth in a universe containing mirror
dark matter. Taking adiabatic scale-invariant perturbations as the input, we
confirm that the resulting processed power spectrum is richer than for the more
familiar cases of cold, warm and hot dark matter. The new features include a
maximum at a certain scale $\lambda_{max}$, collisional damping below a smaller
characteristic scale $\lambda'_S$, with oscillatory perturbations between the
two. These scales are functions of the fundamental parameters of the theory. In
particular, they decrease for decreasing $x$, the ratio of the mirror plasma
temperature to that of the ordinary. For $x \sim 0.2$, the scale
$\lambda_{max}$ becomes galactic. Mirror dark matter therefore leads to
bottom-up large scale structure formation, similar to conventional cold dark
matter, for $x \stackrel{<}{\sim} 0.2$. Indeed, the smaller the value of $x$,
the closer mirror dark matter resembles standard cold dark matter during the
linear regime. The differences pertain to scales smaller than $\lambda'_S$ in
the linear regime, and generally in the non-linear regime because mirror dark
matter is chemically complex and to some extent dissipative. Lyman-$\alpha$
forest data and the early reionisation epoch established by WMAP may hold the
key to distinguishing mirror dark matter from WIMP-style cold dark matter.
