The -nosticky option does not do quite what it sounds like it might. It just avoids the generation of special hidden cgi fields, but does not affect the stickyness of general form fields:
This makes CGI.pm not generating the hidden fields
.submit and .cgifields. It is very useful if you don't
want to have the hidden fields appear in the
querystring in a GET method. For example, a search
script generated this way will have a very nice url
with search parameters for bookmarking.
You're right, in order to avoid stickyness, you must set the value explicity. Either set it to undef or an empty string, or a specific new value in the case you mention above.