Firstly of all, your code will display -no data- when there is data and that data is 0 (zero). Fix:

print OUT defined($in{$_}) ? $in{$_} : '-no data-', "\t";
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Furthermore, your approach won't work. According to the HTML spec (and verified with a few browsers),

A switch is "on" when the control element's checked attribute is set. When a form is submitted, only "on" checkbox controls can become successful.

That means that only checked checkboxes are submitted. defined($in{$_}) will always be true.