First, something truly awful happened to your post and it's all plaintextified-HTML. Yikes! Looks like a runaway <CODE> tag. Don't forget about the "Preview" button, neh?

The HTTP_REFERER(sic) is set by the browser at its own discretion. There are occasions, like using JavaScript to open a window, where the browser decides not to send that information to you at all. If it's not sent, you can take a wild guess as to where they came from, but it's not very reliable unless you have a user-specific cookie and they are using a single window.

Just as a note, you can't set the referrer inside the browser using JavaScript, nor can you tell the browser what the referer "should" be using Perl. It's just one of those things you don't have much control over.