Perfect subsets of invariant CA-sets

The familiar theorem that any $\Sigma^1_2(a)$-set $X$ of real numbers (where $a$ is a fixed real parameter) not containing a perfect kernel necessarily satisfies the condition $X\subseteq\mathbf L[a]$ is extended to a wider class of sets, with countable ordinals allowed as additional parameters in $\Sigma^1_2(a)$-definitions.