perl - 在Perl中，为什么 `while(<HANDLE>) {…}`构造不本地化 `$_`？

Question

Perl不使用以下语法自动本地化$_的设计(或技术)原因是什么:

while (<HANDLE>) {...}

改写为:

while (defined( $_ = <HANDLE> )) {...}

所有其他隐式写入$_的构造都以本地化方式(for/foreach，map，grep)这样做，但是对于while，您必须显式本地化变量:

local $_;
while (<HANDLE>) {...}

我的猜测是，它与在带有命令行开关的“Super-AWK”模式下使用Perl有关，但这可能是错误的。

因此，如果有人知道(或者更好的是参与了语言设计讨论)，您能否与我们分享这种行为背后的原因？更具体地说，为什么尽管允许$_的值在循环外仍然存在，但仍然很重要(我倾向于在S​​O和其他Perl代码中经常看到这种错误)，但为什么它仍然很重要？

如果以上内容不清楚，则在此示例中显示必须用$_本地化while的原因:

sub read_handle {
    while (<HANDLE>) { ... }
}

for (1 .. 10) {
     print "$_: \n"; # works, prints a number from 1 .. 10
     read_handle;
     print "done with $_\n";  # does not work, prints the last line read from
                              # HANDLE or undef if the file was finished
}

Answer 1

在perlmonks.org上的thread中:

There is a difference between foreach and while because they are two totally different things. foreach always assigns to a variable when looping over a list, while while normally doesn't. It's just that while (<>) is an exception and only when there's a single diamond operator there's an implicit assignment to $_.

并且:

One possible reason for why while(<>) does not implicitly localize $_ as part of its magic is that sometimes you want to access the last value of $_ outside the loop.