sub XXX {
    my ($self) = @_;
    $self->{age} = '17';   # does as you say
    $_[0]->{'age'} = '21'; # a miraculous drivers license
    $self = { %{$self} };  # now we have a copy of the innards
    $self->{age} = '17';   # restoration of youth
# the world now thinks age=>21, we have private age=>17
# Youth will be lost when the sub exits
}