$server = "http://www.foo.com"
$path   = "/absolute/path/"
$html   = '
   <a href="/absolute/no/dns">absolute with no dns</a>
   <a href="http://absolute.with/dns.html">http://absolute.with/dns.html</a>
   <a href="relative/without/dns.html">relative/without/dns.html</a>
   <a href="relative2/without/dns.html">relative2 without dns.html</a>
';

$html =~ s/
      (<\s*
         (?:a|img|area)
         [^>]+?(?:href|src)
         \s*=\s*
         ["']?
      )
      (
         [^'"\/>]
         [^'" >]+?
      )
      ([ '"]?>)
      /
      $1.sprintf("%s%s", $path, $2).$3
      /sigex;