The "obvious" answer is that STDERR of the script somehow doesn't work (for example it might not be opened, or opened to a pipe with something hanging at the other end), and something tries to write to STDERR.

You might not notice the output when it goes to STDOUT (maybe it's after the end of the closing body or html tag or somewhere else where your browser doesn't display it?). Or maybe something even tries warn() with an empty string or just a space or something similar.

To test this idea, simply print something to STDERR and see if it also hangs.