Passing variable from block within including template
Anything that is defined in index.twig outside of a block will be available in skeleton.twig, and so also in template.twig. This should work:

skeleton.twig

{% include '_includes/template' %}


index.twig

{% extends "skeleton" %}

{# Set parameter #}
{% set param = {
    section: ['handle1', 'handle2']
} %}