perl - `print $fh <<' EOF 中的 here-doc 问题'` : Perl executing the here doc

Question

(根据 https://stackoverflow.com/a/17479551/6607497 它应该工作，但没有)我有一些这样的代码:

use strict;
use warnings;
if (open(my $fh, '>', '/tmp/test')) {
    print $fh << 'TAG';
    BEGIN {
       something;
    }
TAG
    close($fh);
}

如果我遗漏了 $fh(这是一个为输出而打开的文件句柄，顺便说一句)，BEGIN block 会正确输出(到 STDOUT)。然而，当我添加 $fh 时，Perl (5.18, 5.26) 试图执行 something 导致运行时错误:

Bareword "something" not allowed while "strict subs" in use at /tmp/heredoc2.pl line 6.
syntax error at /tmp/heredoc2.pl line 9, near "FOO
    close"
Execution of /tmp/heredoc2.pl aborted due to compilation errors.

怎么了？

Answer 1

这个问题的细节很有趣(原来的 Perl 是 5.18.2，但是用 5.26.1 作为例子):

首先是一些没有 $fh 的代码:

#!/usr/bin/perl
use strict;
use warnings;
if (open(my $fh, '>', '/tmp/test')) {
    print << 'FOO_BAR';
BEGIN {
    something;
}
FOO_BAR
    close($fh);
}

perl -c说:/tmp/heredoc.pl syntax OK , 但没有任何输出!

如果我添加 $fh之前<< ，我得到这个错误:

Bareword "something" not allowed while "strict subs" in use at /tmp/heredoc.pl line 7.
syntax error at /tmp/heredoc.pl line 10, near "FOO_BAR
    close"
/tmp/heredoc.pl had compilation errors.

最后，如果我删除 'FOO_BAR' 之前的空格, 它有效:

#!/usr/bin/perl
use strict;
use warnings;
if (open(my $fh, '>', '/tmp/test')) {
    print $fh <<'FOO_BAR';
BEGIN {
    something;
}
FOO_BAR
    close($fh);
}

> perl -c /tmp/heredoc.pl 
/tmp/heredoc.pl syntax OK
> perl /tmp/heredoc.pl 
> cat /tmp/test
BEGIN {
    something;
}

也许真正的陷阱是 perlop(1) 中的声明:

           There may not be a space between the "<<" and the identifier,
           unless the identifier is explicitly quoted.  (...)