Modifying your code so I can run this ... I get Illegal value 'invalid_value' for field 'new_animals' at C:/Perl/site/lib/WWW/Mechanize.pm line 541 (you should've mentioned the error message you got).

So I crack open WWW/Mechanize.pm and see this is resulting from a ->value call on a form.

So I check the HTML::Form docs for value and read If the input only can take an enumerated list of values, then it is an error to try to set it to something else and the method will croak if you try.

So what do I do, that's right, I bust out Data::Dumper and discover (via ->find_input)

$VAR1 = bless( {
                 'seen' => [
                             1,
                             0
                           ],
                 'menu' => [
                             undef,
                             'baboon'
                           ],
                 'multiple' => 'multiple',
                 'name' => 'new_animals',
                 'current' => 0,
                 'size' => '10',
                 'type' => 'option',
                 'value_names' => [
                                    'off',
                                    'baboon'
                                  ]
               }, 'HTML::Form::ListInput' );
[download]

Can you guess what I'd try next?

I hope this has been informational :)

update: on a sidenote, for testing purposes I was using the cookie.cgi file from the CGI.pm distribution (i was not calling anyone a baboon)