sometimes I cannot live without knowledge. So I looked into WWW:Mechanize::Cached code, and debugged I have to delete file from cache like this:

my $url = "http://www.rulez.sk/headers.php";
my $req = GET $url, 'Accept-Encoding' => 'identity';
$cache->remove($req->as_string) or print "cannot remove $!";
[download]

I hope helped someone with similar problem :)