my @InvalidFields =
  grep {not defined CGI::param($_) or CGI::param($_) eq ''} CGI::param
+();
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@InvalidFields will now contain the names of any empty fields.

Evaluate @InvalidFields in a scalar context to see how many empty fields there are.

if (@InvalidFields){
  # You have some empty fields, so do something about them
}
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This general technique will work even if you are not using CGI...

Russ