I'd agree with kvale; the standard solution to this problem is to cross-reference the appropriate sections of the other documents with L<...> tags.

If you do want to build a summary POD that includes information from other sources, I think you'll need a pre-processing step with a separate script or template to build it.

I would suggest approaching this by writing your documentation as a template using one of the existing templating tools, like Template::Toolkit or my own Text::MicroMason. The template can include the local POD content and template directives to grep out the relevant bits of the other source files.