LWP has all the tools you need to check the validity of an URL. Commonly, one tries a HEAD request. If that fails try a GET, since some sides deny all HEAD requests.

It's a good idea to contact the webmaster of each site. It is a courtesy, and you can get detailed information about their camera schedule and http practices.

Before you dig in to write code, give some thought to your design. Do you really need to check with each page request? HTTP requests can take a while, do you need to fork off these requests to keep your main script awake? Or the client from timing out?

Some effort spent on realistic study of time and resource needs will really pay off, I think.

After Compline,
Zaxo