Your question is more about FrontPage than it is about Perl, so I'm not sure how well we can help you. The core question is - when you view source on your index.htm file, are you seeing the code you expect to see? Meaning, is the Perl script writing:

<!--webbot bot=\"Includ
+e\" U-Include=\"index_content.htm\" -->
[download]

into your HTML file? If it is, and assuming index_content.htm exists, your problem is with FrontPage. I am especially tempted to assume this since you mention the FrontPage program displays what you expect, while the browser doesn't. Unfortunately, FrontPage isn't standards-compliant, and I don't know much that would help you debug your HTML. All I can say is if the index.htm and index_content.htm files look right to you, then the Perl code isn't your problem.