Wild guess: you are using Apache::Registry and your code looks like this:

my %Params;
....
sub foo {
....
  $page->string($fn, 12, 150, $height, $Params{InvoiceID});
....
}
[download]

In general, this warning means you've created a closure and $Params{InvoiceID} will not change for the rest of the life of your program. That could be bad.