It is not an error but a warning, so you can "hide" it by not activating warnings

In my opinion, hiding warnings is a terrible idea. And the worst way to do it is to just remove use warnings from the code. That stops you from knowing about any warnings that might appear anywhere in your code. That sounds like coding without a safety net for no good reason.

If you do have a piece of code where you want to "hide" a warning then just hide that specific warning in that specific piece of code. In this instance, that might look a bit like this:

{
  no warnings 'uninitialized';

  print start_html(...);
}
[download]

That way you still get the benefit of use warnings throughout most of your code.