After looking at the (5.8.8) source code, I'd have to say that in your case I would first insure that I was using a valid form number. Then I would check your "fields" hash to make sure it is good. It looks like it expects a hash. Your best bet would be to look at line 1492 of /usr/lib/perl5/site_perl/5.8.5/WWW/Mechanize.pm on your computer.

From Mechanize.pm (5.8.8): Notice the "or die" code.

sub submit_form {
    my( $self, %args ) = @_ ;

    for ( keys %args ) {
        if ( !/^(form_(number|name)|fields|button|x|y)$/ ) {
            $self->warn( qq{Unknown submit_form parameter "$_"} );
        }
    }

    if ( my $form_number = $args{'form_number'} ) {
        $self->form_number( $form_number ) or die;
    }
    elsif ( my $form_name = $args{'form_name'} ) {
        $self->form_name( $form_name ) or die;
    }

    if ( my $fields = $args{'fields'} ) {
        if ( isa( $fields, 'HASH' ) ) {
            $self->set_fields( %{$fields} ) ;
        } # TODO: What if it's not a hash?  We just ignore it silently
+?
    }

    my $response;
    if ( $args{button} ) {
        $response = $self->click( $args{button}, $args{x} || 0, $args{
+y} || 0 );
    }
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