As long as you don't check the result of open(), the most likely explanation is that it fails and therefore does not produce a file handle valid for reading.

Try adding or die "Open failed '$mailcheck': $!" immediately after open(HANDLE, $mailcheck) and you'll most likely see the root cause of the problem.

Umm.... a quick update: I just noticed the close() statement inside the read loop. Probably not the best place for it.