That confirms that the server is giving you garbage. It's saying that the zip file is really an UTF-8 HTML document.

'content-type'        => 'text/html; charset=utf-8',
'content-disposition' => 'attachment;filename=auto_20090318_0610.zip',
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The solution is to fix the response received from the web server.

BEGIN {
    my $old_make_request = WWW::Mechanize->can('_make_request');

    no warnings 'redefine';
    *WWW::Mechanize::_make_request = sub {
        my $response = $old_make_request->(@_);

        my $type  = $response->header('Content-Type');
        my $dispo = $response->header('Content-Disposition');

        $response->header('Content-Type' => 'application/zip')
            if defined($dispo) && $dispo =~ m{\.zip$}
            && defined($type)  && $type  =~ m{^text/};

        return $response;
    };
}
[download]

Untested.