 |  |
One operator that can return undef is the line-input operator, <STDIN>. Normally, it will return a line of text. But if there is no more input, such as at end-of-file, it returns undef to signal this.[66] To tell whether a value is undef and not the empty string, use the defined function, which returns false for undef, and true for everything else:
[66]Normally, there's no "end-of-file" when the input comes from the keyboard, but 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
|  | |
| 2.11. The undef Value |  | 2.13. Exercises |
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