All right, this time I'm not going to just hand you an answer - I'm going to try to make you work for it ;) You've asked a lot of questions about cookies and form handling at pm, and seem to be asking some of the same here again. Here are some clues that will hopefully point you in the right direction...

#1: What happens when you set a cookie with an empty value? Hint - it doesn't get stored, and if there is already a cookie with that name...

#2: What happens when you compare two undefined values?

perl -wle 'print "true!" if undef eq undef';
# hmm, undef eq undef is true!
[download]

#3: What happens when you print HTTP headers more than once to the browser?

I think two out of three of these questions are answered at Ovid's CGI course mentioned above (I can't reach it from here right now). Also try perldoc CGI, then have a look at CGI Programming with Perl (one of many good O'Reilly books). Good luck!