If you're using CGI.pm, it looks like calling param() for an unsubmitted parameter returns undef. The snippet I used to test this is:

print "Undefined parameter: >>";
print defined ($q->param('undefined')) ? "defined" : "not defined";
print "<<\n<p>";
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(Obviously, there's more to it than that, but anyone who knows enough to ask this question ought to know what else to add.)