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...