my $m = new WWW::Mechanize;
my $ROOT = "/path/to/download/directory";

sub visit {
  my $url = shift;
  $m->get($url);
  ...
  for my $link ($m->links) {
    visit($link->url_abs); # problem is here
  }
}
visit("file:$ROOT/index.html");

##</code><code>##

<a href="/foo"></a>
  -- $link->url_abs is "file:/foo"
<a href="../bar"></a>
  -- $link->url_abs is "file:/path/to/download/bar"