Without knowing too much about the problem, it looks like you want something like this:

  my @files;
  foreach $inst (@instances) {
    my @junk = $inst->look_down(
                 'tag'   => 'span',
                 'class' => 'inst',
                 sub {
                   my $result = $_[0]->look_down(
                     '_tag' => 'a',
                     'href' => qr/(\w+\.html)#\w/));
                   push @files => $1 if $1;
                   return $result;
                 });
[download]

Of course, this falls into the "wild-assed guess" category. It seems straightforward enough that I'm wondering if I've misunderstood something.