I have no idea what I was thinking when I wrote the first part of that post. Like I said in the second half, unchecked checkboxes are not in the list of params. As per the HTML spec, they are not sent by the client to the server. You need to loop over the fields you want to output, not over the params

    my @fields = qw(
       ...
       c_...
       c_...
       c_...
       ...
    );

    foreach (@fields) 
    {    
        $in{$_} = $query->param($_);    

        if (/^c_/ && defined($in{$_})) {
           $check{$_} = "CHECKED";
        }

        ...
    }
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