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Section 2.11.  The defined Function

 
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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

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