perl - 哈希中的加号是什么意思？

Question

我有以下 Perl 代码，但我不明白它在做什么。

use constant ANIMAL => 'rabbit'; 
if ($self->{+ANIMAL}) {
    # Do something here
}

+有什么用常量前的符号 ANIMAL意思？

Answer 1

来自 perldoc constant :

You can get into trouble if you use constants in a context which automatically quotes barewords (as is true for any subroutine call). For example, you can't say $hash{CONSTANT} because CONSTANT will be interpreted as a string. Use $hash{CONSTANT()} or $hash{+CONSTANT} to prevent the bareword quoting mechanism from kicking in. Similarly, since the => operator quotes a bareword immediately to its left, you have to say CONSTANT() => 'value' (or simply use a comma in place of the big arrow) instead of CONSTANT => 'value'.