In theory... is it possible to
parse a POST payload in which a url is supplied,
then use the supplied url to perform a GET to acquire data,
that data is used to query a db and
then send my response to the POST?
Or will my original POST request pick up the GET response and kill everything?
I can't seem to find an example or reference to this being done anywhere...
Yes, it's possible, and not just in theory. I've done much the same thing myself. Just use
LWP (or your preferred HTTP client library) to perform the
GET as usual. Whether you're doing it within a
POST handler or not makes no difference.