C<undef>. A successful system or library call does B<not> set
the variable to zero.
-If used an a string, yields the corresponding system error string.
+If used as a string, yields the corresponding system error string.
You can assign a number to C<$!> to set I<errno> if, for instance,
you want C<"$!"> to return the string for error I<n>, or you want
to set the exit value for the die() operator. (Mnemonic: What just