current-approach.pl

my $form = HTML::Form->parse($mech->response);

my $dropdown = $form->find_input('DropdownName');

my @labels = $dropdown->value_names;
my @values = $dropdown->possible_values;

# The above works perfectly for fetching a list of values and labels, 
# but for each entry in the drop-down, I need to know which <optgroup> 
# it's in.  HTML::Form doesn't appear to provide that info...

example.html

<select name="fieldname">
  <optgroup label="Optgroup One">
    <option value="1">Foo</option>
    <option value="2">Bar</option>
  </optgroup>
  <optgroup label="Optgroup Two">
    <option value="3">Foo</option>
    <option value="4">Bar</option>
  </optgroup>
</select>