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