  Commutative post-Lie algebra structures on Lie algebras, in short CPA
structures, have been studied over fields of characteristic zero, in particular
for real and complex numbers motivated by geometry. A perfect Lie algebra in
characteristic zero only admits the trivial CPA-structure. In this article we
study these structures over fields of characteristic $p>0$. We show that every
perfect modular Lie algebra in characteristic $p>2$ having a solvable outer
derivation algebra admits only the trivial CPA-structure. This involves a
conjecture by Hans Zassenhaus, saying that the outer derivation algebra ${\rm
Out}(\mathfrak{g})$ of a simple modular Lie algebra $\mathfrak{g}$ is solvable.
We try to summarize the known results on the Zassenhaus conjecture and prove
some new results using the classification of simple modular Lie algebras by
Premet and Strade for algebraically closed fields of characteristic $p>3$. As a
corollary we obtain that that every central simple modular Lie algebra of
characteristic $p>3$ admits only the trivial CPA-structure.
