Sorry, can't help much from here since I've never tried to install a non-system perl.If you can get the second perl working, I was going to say you'd install the xkcd reader directly from cpan using one of the cpan clients (cpan, cpan plus, cpan minus, etc. -- okay, I'm not sure there really is an "etc." here but probably). But looking at the port, it doesn't look like the viewer on cpan. Compare
http://www.autistici.org/fox/files/XKCD_viewer/README (the one the port uses)
to
http://search.cpan.org/~woldrich/App-xkcd-0.082/xkcd (an app on cpan I assumed for a moment the port must be based on)
Looks like different programs. Maybe instead of going down the route of getting a second perl you could try installing this other xkcd viewer directly from cpan. It doesn't seem to have as difficult prerequisites.