hmm.. looking briefly at source, that error comes from HTML::Form, but i didn't see a way to explicitly provide the possible_values ...

At worst (assuming a GET is acceptable), instead of the submit_form() call you could just do (using a module to construct the query string if you like; needs escaping as well):

my %fields      = (
            search      => 'new',
            make        => 'Subaru',
            model       => 'B9 Tribeca',
            trim        => '5-Passenger Ltd.',
            showImages  => 'yes',
            fz          => 'true',
        );
$agent->get( $agent->current_form->agent . '?' . join("&", map { $_ ."
+=".$fields{$_} } keys %fields) );
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