I know of no serious module that could resolve a relative URL into a full URL. There are millions of web sites; how would you choose which one to use? Would index.pl?node=100 apply to Perl Monks, Everything 2, or Anime-Fu?

Once you answer that question, it's not difficult to write a bit of code to absolutify relative URIs. I suggest the URI module. If there's no scheme and hostname, add the default.