Mapping in this case simply means the routing or the redirection of a request. I included a dispatch table in the code:

        my %routing = (
                '/submit' => $form_validator,
                '/' => $form,            
                );
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When you submit the HTML form, it is sent to the URL '/submit' as defined by the HTML form attribute 'action'. Since '/submit' is associated with $form_validator, the anonymous function that the $form_validator refers to is invoked.