Learning Perl: 2.11. The defined Function

2.11. The defined Function

One operator that can return undef is the line-input operator,<STDIN>. Normally,it returns a line of text. But if there is no more input,such as at end-of-file,it will return undef to signal this.^[*] To tell if a value is undef and not the empty string,use the defined function,which returns false for undef and true for everything else:

^[*] Normally,there's no "end-of-file" when the input comes from the keyboard,but the input may have been redirected to come from a file. Or the user may have pressed the key that the system recognizes to indicate end-of-file.

    $madonna = <STDIN>;
    if ( defined($madonna) ) {
      print "The input was $madonna";
    } else {
      print "No input available!/n";
    }

If you'd like to make your own undef values,you can use the obscurely named undef operator:

    $madonna = undef; # As if it had never been touched