If you read the manual, the argument to -script should be the text of your Javascript code; the start_html method will wrap it in appropriate <script> tags for you.

-script should point to a block of text containing JavaScript function definitions. This block will be placed within a <script> block inside the HTML (not HTTP) header.

What you're probably seeing is the extra straggling content which has been wrapped around your script section (CGI.pm's starts it; your start is ignored; yours closes the script tag; CGI.pm's comment-to-hide-from-old-browsers line is shown, and it's extra ending </script> is hidden).