为何Perl三元条件运算符处理未定义哈希引用键与普通变量行为不同？

为什么Perl的三元条件运算符处理未定义哈希引用键时，和处理普通未定义变量的行为不一样？比如当$form未定义但$str已定义时，表达式($str) ? $form = $str : $form = $form;总能把$str的值赋给$form；但换成未定义的哈希键后，这个表达式就不生效了。

我写了一个脚本演示这个异常行为，所有测试里的“Use of uninitialized value”警告是故意保留的，因为我就是在测试未定义值的情况。

#!/usr/bin/env perl
# ternary operator
# condition ? if True : if False 
# Why does the ternary operator behave different with a undefined hashref key
# than a regular undefined $ variable ?

use strict;
use warnings;
use diagnostics;

my $i = 1;
my $form = { vc => 'customer', customer_id => ''};
my $ref = { customer_id => 12345 };

&string_test;# passes
print "-" x 80;
&mixed_test_ref; # passes
print "-" x 80;
&mixed_test; #fails
print "-" x 80;
&hashref_test; #fails
print "-" x 80;
&works_always; 
print "-" x 80;
print "\n";

sub string_test {
    my $str = '1234';
    my $form; # not defined
    print qq|
Why does the ternary operator behave different
with a undefined hashref key
than a regular undefined \$ variable ?
First with a undefined variable called form:   
   Before:
        str : '$str' \n
        form : $form
    |; # will throw undefined warning


    # ternary operator
    #condition ? if True : if False  
    ($str) ? $form = $str : 
    $form = $form;

    print qq|
       Always works:
        str : '$str' \n
        form: $form \n|;

}


sub mixed_test_ref {

    my $local;
    print qq|
    Mixed test ref
    Before:
        form->{'vc'} '$form->{'vc'}' \n
        ref->{"$form->{'vc'}_id"} : '$ref->{"$form->{'vc'}_id"}' \n
    |;


    # ternary operator
    #condition ? if True : if False  
    ($ref->{"$form->{'vc'}_id"}) ? $local = $ref->{"$form->{'vc'}_id"} : 
    $local = $local;

    print qq|
        After:
        form->{'vc'} '$form->{'vc'}' \n
        ref->{"$form->{'vc'}_id"} : '$ref->{"$form->{'vc'}_id"}' \n
         local: '$local' \n
    |;

    if ($local = $ref->{"$form->{'vc'}_id"}) {print "Pass\n"}
    else {print "Fail\n"}
}

sub mixed_test {
    # $form->{"$form->{'vc'}_id_$i"} is not defined
    # setting it to empty string makes it pass
    #$form->{"$form->{'vc'}_id_$i"} = '';
    delete $form->{"$form->{'vc'}_id_$i"};
    my $str = '1234';
    
    print qq|
    mixed test
    Before:
        str : '$str' \n
        form: '$form->{"$form->{'vc'}_id_$i"}' \n
    |;


    # ternary operator
    #condition ? if True : if False  
    ($str) ? $form->{"$form->{'vc'}_id_$i"} = $str : 
    $form->{"$form->{'vc'}_id_$i"} = $form->{"$form->{'vc'}_id_$i"};



    if ( $form->{"$form->{'vc'}_id_$i"} == $str ) {
    print qq|Pass\
        After:
        str : '$str' \n
        form: $form->{"$form->{'vc'}_id_$i"} \n|;
    }
    else {print "Fail\n"}

}

sub hashref_test {
    # $form->{"$form->{'vc'}_id_$i"} is not defined
    # setting it to empty string makes it pass
    #$form->{"$form->{'vc'}_id_$i"} = '';
    delete $form->{"$form->{'vc'}_id_$i"};
    print qq|
    hash ref test
        Before:
        form->{'vc'} '$form->{'vc'}' \n
        ref->{"$form->{'vc'}_id"} : '$ref->{"$form->{'vc'}_id"}' \n
        form->{"$form->{'vc'}_id_$i"} : $form->{"$form->{'vc'}_id_$i"} \n
    |;


    # ternary operator
    #condition               ? if True : if False  
    $ref->{"$form->{'vc'}_id"} ? $form->{"$form->{'vc'}_id_$i"} = $ref->{"$form->{'vc'}_id"} : 
    $form->{"$form->{'vc'}_id_$i"} = $form->{"$form->{'vc'}_id_$i"};

    if ($form->{"$form->{'vc'}_id_$i"}) {print "Passes and I am amazed\n"}
    else {
        print qq|
            Why does this not work?
            After:
            form->{'vc'} '$form->{'vc'}' \n
            ref->{"$form->{'vc'}_id"} : '$ref->{"$form->{'vc'}_id"}' \n
            form->{"$form->{'vc'}_id_$i"} : $form->{"$form->{'vc'}_id_$i"} \n|;
    }

}

sub works_always {
    if ($ref->{"$form->{'vc'}_id"}) { $form->{"$form->{'vc'}_id_$i"} = $ref->{"$form->{'vc'}_id"} }
    # not necessary but for completeness
    else { $form->{"$form->{'vc'}_id_$i"} = $form->{"$form->{'vc'}_id_$i"} }

    print qq|
        Always works as expected with if statement:
        form->{'vc'} '$form->{'vc'}' \n
        ref->{"$form->{'vc'}_id"} : '$ref->{"$form->{'vc'}_id"}' \n
        form->{"$form->{'vc'}_id_$i"} : $form->{"$form->{'vc'}_id_$i"} \n|;
}

这根本不是哈希键和普通变量的差异问题，而是运算符优先级导致的解析错误。

Perl中，三元运算符?:的优先级高于赋值运算符=，所以你写的表达式：

($str) ? $form->{"key"} = $str : $form->{"key"} = $form->{"key"};

会被Perl解析成：

( ($str) ? ($form->{"key"} = $str) : $form->{"key"} ) = $form->{"key"};

当$str为真时，三元表达式的第一部分$form->{"key"} = $str会执行，它的返回值是$str。然后Perl会尝试把$form->{"key"}（此时还未在哈希中创建，值为undef）赋值给这个返回值——但$str是字符串常量，不是左值，这个赋值操作会被忽略，导致哈希键最终没有被设置。

而在普通变量的场景中，$form是左值，所以即使解析成(($str) ? ($form = $str) : $form) = $form，赋值给左值是允许的，最终$form会被两次赋值（第一次是$str，第二次是$form自己，结果还是$str），所以看起来“正常工作”，但这其实是巧合，写法本身就是错误的。

正确的写法 #

你应该把赋值操作放在三元运算符的外面，让三元运算符返回要赋值的值：

$form->{"$form->{'vc'}_id_$i"} = $str ? $str : $form->{"$form->{'vc'}_id_$i"};

或者更简洁的写法（利用Perl的短路特性）：

$form->{"$form->{'vc'}_id_$i"} ||= $str;

如果只是想在目标值未定义时赋值，也可以用：

$form->{"$form->{'vc'}_id_$i"} = $str unless defined $form->{"$form->{'vc'}_id_$i"};

另外，你脚本里的works_always子函数用if语句能正常工作，是因为if的代码块不存在优先级问题，赋值操作是明确在条件为真时执行的。

内容的提问来源于stack exchange，提问作者turtle