There is no null keyword in Perl.

The value you're most likely thinking of is undef, and that's what $cgi->param(...) returns if no such field is found. So most likely, the below code already does what you might want:

    $fieldname = $cgi->param('fieldname')
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Note that the browser might send empty strings instead of missing values, depending on how your form is set up. But that's a different problem.