使用定义与Filehandle和while循环(Usage of defined with Filehandle and while Loop)

 在阅读关于高级Perl编程（1）的书时，我遇到了这样的代码： 
 
while (defined($s = <>)) {
    ...
 
 在这里使用defined有什么特别的理由吗？ perlop的文档说： 
 
 
 
 在这些循环结构中，分配的值（无论赋值是自动的还是显式的）都会被测试以确定它是否被定义。 定义的测试避免了行中有一个字符串值被Perl视为假的问题，例如""或没有尾随换行符的"0" 。 如果你真的想要这样的值来终止循环，应该明确地测试它们：[...] 
 
 
 那么，会不会有一个角落案例，或者仅仅是因为这本书太旧了，自动defined测试是在最近的Perl版本中添加的？ 
 
 
 （1） 高级Perl编程，第一版，Sriram Srinivasan。 O'Reilly（1997） 

While reading a book on advanced Perl programming(1), I came across this code:
 
while (defined($s = <>)) {
    ...
 
Is there any special reason for using defined here? The documentation for perlop says:
 
 
 
In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where line has a string value that would be treated as false by Perl, for example a "" or a "0" with no trailing newline. If you really mean for such values to terminate the loop, they should be tested for explicitly: [...]
 
 
So, would there be a corner case or that's simply because the book is too old and the automatic defined test was added in a recent Perl version?
 
 
(1) Advanced Perl Programming, First Edition, Sriram Srinivasan. O'Reilly (1997)

原文：https://stackoverflow.com/questions/9621221