Stringificaton运算符意外调用

Question

我在MPEG::Audio::Frame中调试a test。如果我运行这个测试，我会得到：

$ cpan -g MPEG::Audio::Frame
$ tar zxvf MPEG-Audio-Frame-0.09.tar.gz
$ cd MPEG-Audio-Frame-0.09
$ perl Makefile.PL
$ make
$ perl -I./blib/lib t/04-tie.t 
1..5
ok 1 - use MPEG::Audio::Frame;
ok 2 - 'tie' isa 'MPEG::Audio::Frame'
Not a HASH reference at blib/lib/MPEG/Audio/Frame.pm line 273, <DATA> line 1.
# Looks like your test exited with 255 just after 2.

我将问题缩小到以下最小示例：

package My::Module;
use feature qw(say);
use strict;
use warnings;
use overload '""' => \&asbin;

sub asbin {
    my $self = shift;
    $self->{binhead}  # $self is not yet a hash, so execution stops here.
}

sub TIEHANDLE {
    bless \$_[1], $_[0]
}

sub READLINE {}

sub read {
    say "reading..";
    my $pkg = shift;
    my $fh = shift || 0;  # Why is the stringification operator called here?
}

package main;
use feature qw(say);
use strict;
use warnings;

tie *FH, 'My::Module', *DATA;
My::Module->read(\*DATA); 
<FH>;

__DATA__
abc

为什么为语句My::Module->read(\*DATA)调用字符串化运算符？

Answer 1

shift || 0希望将shift中的参数强制转换为标量。没有为My::Module定义boolify或numify函数重载，因此Perl将使用您的stringify函数。

为了避免在标量上下文中计算对象，您可以将其重新表述为

my $fh  = @_ ? shift : 0;

$fh = shift;
$fh = 0 unless ref($fh) || $fh;

或者定义一个bool函数重载。