Maybe I'm a little too picky here, but I want to encourage everybody to write "right" HTML; with XHTML in mind, of course.. I edited merlyn's code so it creates XHTML conform output:

sub hash_to_html ($;$)
{
    my $x = shift;
    my $i = @_ ? shift : '';

    return "" unless ref ($x) eq "HASH";

    return "$i<ul>\n"
    . join ('', map { "$i  <li>$_</li>\n" . hash_to_html ($x->{$_}, "$
+i    ") } sort (keys (%$x)))
    . "$i</ul>\n";
}
[download]

Which prints (with the input above):

<ul>
  <li>Cows</li>
    <ul>
      <li>Brown</li>
      <li>Green</li>
      <li>Orange</li>
      <li>Strawberry</li>
        <ul>
          <li>Solid</li>
          <li>Spotted</li>
        </ul>
    </ul>
  <li>Dogs</li>
    <ul>
      <li>Purple</li>
        <ul>
          <li>Solid</li>
          <li>Spotted</li>
        </ul>
    </ul>
</ul>
[download]

Regards,
-octo