use constant, Perl question

[Gary Yang]

I have hard time of understanding “use constant”. I do not understand why place 
the “+” in front of constant variable. See the code below. In the for loop, It 
adds “+” (+kAWSAccessKeyId,). In the new function call, it adds the “+” 
(+RequestSignatureHelper::kAWSAccessKeyId => myAWSId). I read the perldoc of 
“use constant”. But, no clue. Can someone explain what it mean? Or point me any 
books or URLs I can read.  I got the code below from some samples. If this 
mailing list is not the proper place to ask this question, please tell me which 
mailing list is best for this question.  Thanks.



use constant kAWSAccessKeyId => 'AWSAccessKeyId';
use constant kAWSSecretKey   => 'AWSSecretKey';
use constant kEndPoint       => 'EndPoint';
use constant kRequestMethod  => 'RequestMethod';
use constant kRequestUri     => 'RequestUri';


sub new {

my $self = {};

......

    for (+kAWSAccessKeyId, +kAWSSecretKey, +kRequestMethod, +kRequestUri) {
        $self->{$_} = $args{$_}
    };

......
$self->{+kRequestMethod} 

......

}

my $helper = new RequestSignatureHelper (
    +RequestSignatureHelper::kAWSAccessKeyId => myAWSId,
    +RequestSignatureHelper::kAWSSecretKey => myAWSSecret,
    +RequestSignatureHelper::kEndPoint => myEndPoint,
);