$!      If used numerically, yields the current value of the C "errno"
               variable, or in other words, if a system or library call fails,
               it sets this variable.  This means that the value of $! is
               meaningful only immediately after a failure:

                   if (open(FH, $filename)) {
                       # Here $! is meaningless.
                       ...
                   } else {
                       # ONLY here is $! meaningful.
                       ...
                       # Already here $! might be meaningless.
                   }
                   # Since here we might have either success or failure,
                   # here $! is meaningless.