Yes, you use file:// where you would otherwise use http://.

If you see your CPAN mirror using http://my-ip-addres-vbox in the browser, then the web server configuration you've shown does not match the actual configuration:

<VirtualHost localhost:80>
[download]

This means that the web server will only run in your virtual box and only listen to the localhost interface there.

Do you want to make the CPAN mirror available within that virtual box, or to machines outside of that virtual box? If you want to make it available outside of that virtual box, file:// will not work. Then you will need to use http://, but the web server configuration you've shown does not look as if the web server (or rather, the CPAN mirror part of that webserver) would be reachable from outside that virtual box.