Yes, Merlyn, but I became confused and not as a result of anything you wrote.
In fact, you do say in the article "URL_html contains the same name as $DIR_html, but mapped into the webserver's space so that we can redirect the client's browser to the results file."
The lesson here is that every static web page has a dual nature. It possesses a physical path in the file system, and it possesses a URL under the http protocol.
Clearly, it would have been a mistake for me to attempt to try to pass a URL to IO::File->new, which is what my proposed "correction" would have done.