The thing is, you're writing POD, not HTML. POD does not really have a concept of relative links, it just has "links". The pod translator has to figure out how
to link stuff, and it can do that by making (relative) urls for HTML, creating hyperlinks in for info files or just displaying the raw text. Full URLs in links are useful because they will work more or less anywhere, independent of the location of the POD or generated file. Relative links depend on the eventual location of the generated file, which is not known in general. Ofcourse, you can always write your own pod2html tranlator if you do know.