为什么选择在单独的语句中声明和初始化词法变量？

Question

这是AnyEvent::Intro

的摘录

# register a read watcher
my $read_watcher; $read_watcher = AnyEvent->io (
    fh   => $fh,
    poll => "r",
    cb   => sub {
        my $len = sysread $fh, $response, 1024, length $response;

        if ($len <= 0) {
           # we are done, or an error occurred, lets ignore the latter
           undef $read_watcher; # no longer interested
           $cv->send ($response); # send results
        }
    },
);

为什么使用

my $read_watcher; $read_watcher = AnyEvent->io (...

而不是

my $read_watcher = AnyEvent->io (...

Answer 1

因为闭包引用$read_watcher并且$read_watcher解析为词法的范围仅以包含my 之后的语句开头。

这是故意的，所以这样的代码引用了两个独立的变量：

my $foo = 5;

{
    my $foo = $foo;
    $foo++;
    print "$foo\n";
}

print "$foo\n";