opps.. my mistake in the sample
but in my code it has the correct syntax
$pjx = new CGI::Ajax ('func' => \&func);

coz it execute seccussfuly, on the first time but in second time, it complains about javascript "previous_value"

my code is pretty long, so, I made a simplified version
I tried it several time but ended up using workaround using
hidden input to keep the previous value