defined 和exists函数

在hash中，当检验一个元素值是否被定义是用defined，当检验一个key在hash中是否存在时，用exists。exists 和 defined 的差异及用法示例：

$hash{"test"}='what';

if (exists $hash{"test100"}){
    print "test100";
}

if (defined $hash{"test"}){
    print "yes test";
}

yes test

defined

Returns a Boolean value telling whether EXPR has a value other than the undefined value . If EXPR is not present, $_ will be checked.

exists

Given an expression_r_r that specifies a hash element or array element, returns true if the specified element in the hash or array has ever been initialized, even if the corresponding value is undefined.