# $link_target and $url_just_fetched are known...

my $full_target;

if ( $link_target =~ m{^/} ) {
   # initial "/" means "relative to doc-root":
   my $docroot = $url_just_fetched;
   $docroot =~ s{(?<=[^/]/)[^/].*}{}; # delete everything after first single slash
   $full_target = $docroot . $link_target;
}
elsif ( $link_target !~ /^http/ ) {
   # it's presumably relative to the url just fetched, so:

   if ( $url_just_fetched =~ m{/$} ) {
      $full_target = $url_just_fetched . $link_target;
   }
   # this is the tricky part:
   elsif ( $url_just_fetched =~ /[?&=;:]|\.htm/ )  # probably not a directory name...
   {
      my $last_slash = rindex( $url_just_fetched, "/" ) + 1;
      $full_target = substr( $url_just_fetched, 0, $last_slash ) . $link_target;
   }
   else  # assume its a directory name {
      $full_target = join "/", $url_just_fetched, $link_target;
   }
}
# (if $link_target does start with "http", then it's probably complete already)

# last step:

$full_target =~ s/\#.*//;  # in case the link target is a named anchor within a page