...
 <span class=inst>
   <a href="file23.html#some_tag">...</a>
 </span>
 ...

##</code><code>##

  foreach $inst (@instances) {
    my @junk = $inst->look_down(
                 'tag'   => 'span',
                 'class' => 'inst',
                 sub {
                   $_[0]->look_down(
                     '_tag' => 'a',
                     'href' => qr/(\w+\.html)#\w/));
                     #             ^^^^^^^^^                         
                 });