I haven't done much work with XML (although I understand the basics), so I'm looking for suggested XML modules to get this work done.

A good place to look for a comparison of the various XML documents you might want to look at the Perl-XML FAQ

Any suggestions on what I might use to a) markup and send the HTTPS POST (XML) connection

Here is one way to do it. (well the posting part anyhow);

and b) accept/parse the reply?

Personally, I tend to veer toward XML::Simple for most of my XML needs.

Good Luck

-enlil