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#!/usr/bin/perl -wTR
use strict;
...
print $gateway $expiry;       # ALLOWED

$foo++;       # Still of CreditCard type...

restrict CreditCard, CreditCardGateway;
filter CreditCard, \&clean_credit_card;
...
    $restricted = s/.{12}/./;  # replace 12 digits with .s 
    return $restricted;
}