So I added override => 1 and now everything is working.

Ah, that makes sense. If you had previously submitted the form with that specific checkbox being unchecked, then the normal behavior of CGI.pm is to keep that existing state when it prints the new copy of the HTML -- that would explain why the results with no input using my code didn't match what you were seeing.

To make sure that your form is always in the "pristine" state rather than inheriting from the input parameters it received, then you have to tell CGI.pm to do it that way.

The CGI.pm documentation for 4.51 explains this in the CGI::HTML::Functions "CREATING FILL-OUT FORMS:" section (as does 4.60 in the equivalent section of the docs):

Another note The default values that you specify for the forms are only used the first time the script is invoked (when there is no query string). On subsequent invocations of the script (when there is a query string), the former values are used even if they are blank.

It then goes on to explain that you can either use the param() method to set the value for that parameter to what you want, or you can use the override flag to have the "default" values you supply in your code used even if there was an input value from the query string or post parameters.