Perl:意外的 $_ 行为

Question

use Modern::Perl;
use DateTime;
use autodie;

my $dt;

open my $fh, '<', 'data.txt';

# get the first date from the file
while (<$fh> && !$dt) {
   if ( /^(\d+:\d+:\d+)/ ) {
      $dt = DateTime->new( ... );
   }
   print;
}

我期待这个循环读取文件的每一行，直到读取第一个日期时间值。

取而代之的是 $_ 被统一化，我收到了大量“模式匹配中的未初始化值 $_”(和打印)消息。

任何想法为什么会发生这种情况？

一种

Answer 1

$_仅在您使用表单 while (<$fh>) 时才设置形式，而你不是。

看这个:

$ cat t.pl
while (<$fh>) { }
while (<$fh> && !$dt) { }

$ perl -MO=Deparse t.pl
while (defined($_ = <$fh>)) {
    ();
}
while (<$fh> and not $dt) {
    ();
}
t.pl syntax OK

来自 perlop文档:

Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a while statement (even if disguised as a for(;;) loop), the value is automatically assigned to the global variable $_, destroying whatever was there previously.