restrict CreditCard, CreditCardGateway;
filter CreditCard, \&clean_credit_card;

my CreditCard $credit_card = $cgi->param("credit_card");
print $credit_card;        # ALLOWED (filters card)

my CreditCardGateway $gateway;
open ($gateway, "| cc_card_gateway") or die "failed to open gateway: $!";

print $gateway $credit_card;  # ALLOWED (prints full 
                               # details)

# very very naive cleaning function
sub clean_credit_card {
    my ($restricted) = @_;

    $restricted = s/.{12}/./;  # replace 12 digits with .s 
    return $restricted;
}