You have another problem waiting in the wings once you solve your immediate problem. That is, if you're doing several existence checks (using LWP::Simple or whatever) while the end user's browser is waiting, you risk timing out the browser.

There are two approaches to this. One is to arrange emit the HTML stream in a dribble, emitting each WebCam URL as you've done the verification. There's still the risk that the verification will time out, causing a cascading failure to the end user's browser.

The second approach is to periocially scan your WebCam URLs, caching the results. Then feed the browser from the cache. Your eliminate the risk of timeout by adding a (smaller?) risk that a webcam will have become unavailable since the last time you scanned it.