debugging output: the stuff that is generated by your browser. In particular, look for "error messages" in the JavaScript console. A huge practical problem of JavaScript is that, instead of "bothering the user with" any sort of error which occurs in the web-site's code,it simply buries it in a console-log that ordinary users will never see, and then "just keeps going." (Or: "stops, but without complaining.") And, by the way, "none of this is Perl-specific."