What does a plus sign mean in a hash?

Question

I have the following piece of Perl code, but I can't understand what its doing.

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

What does the + sign before the constant ANIMAL mean?

Answer 1

From 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'.