Perl匿名哈希添加新键值对时出现编译错误问题

嘿，我一眼就瞅到你代码里的问题啦！你遇到的编译错误，根源是变量声明顺序搞反了——你在使用$design这个哈希引用之前，还没来得及用my声明它呢！

错误代码的核心问题 #

看你这段代码的执行顺序：

my $selected = 'box'; 
my $d = $design->{$selected}; # 这里$design还没被声明，Perl根本不知道它是什么！
my $design = { box => { ne => {data => 'north-east'}, nw => {data => 'north-west'}, n => {data => 'north'}, s => {data => 'south'}, e => {data => 'east'}, w => {data => 'west'}, se => {data => 'south-east'}, sw =>... } };

Perl里用my声明的词法变量，只有在声明语句之后才会在当前作用域生效。你提前调用$design，Perl会判定这是一个未声明的变量，直接抛出编译错误。

修正后的代码 #

只需要把$design的声明移到使用它的语句之前就行：

my $selected = 'box';
# 先声明并初始化$design哈希引用
my $design = { 
    box => { 
        ne => {data => 'north-east'}, 
        nw => {data => 'north-west'}, 
        n => {data => 'north'}, 
        s => {data => 'south'}, 
        e => {data => 'east'}, 
        w => {data => 'west'}, 
        se => {data => 'south-east'}, 
        sw => {data => 'south-west'} # 补全你省略的内容
    }
};
# 现在可以正常访问$design的键值对了
my $d = $design->{$selected};

额外小提示 #

如果之后还要给这个哈希引用添加新的键值对，直接用$design->{new_key} = new_value;的格式就可以，比如：

# 给box添加一个新的方向键
$design->{box}->{center} = {data => 'center'};

内容的提问来源于stack exchange，提问作者Martin Kenley