$OS_ERROR
       $ERRNO
       $!      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 vari-
               able.  This means that the value of $! is meaning-
               ful 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.

               In the above meaningless stands for anything:
               zero, non-zero, "undef".  A successful system or
               library call does not set the variable to zero.

               If used as a string, yields the corresponding sys-
               tem error string.  You can assign a number to $!
               to set errno if, for instance, you want "$!" to
               return the string for error n, or you want to set
               the exit value for the die() operator.  (Mnemonic:
               What just went bang?)

               Also see "Error Indicators".