# example of constant:

   # use constant TMPDIR = '/bigdrive/bigbucket01';
   use constant TMPDIR = '/tmp'; # /tmp now mapped to above

# example of Readonly:
sub Write {
    my $self = shift;
    Readonly my %reset = %$self;
    # ... code that modifies $self
    # e.g. with 'dangerous' use of eval ...
    $self -> ( STATE } = eval $self -> { COMMAND }
        or goto ERROR;
    # ...
    $self -> Close() and return 1;

    # if error reset $self:
ERROR:
    delete $self -> { $_} for keys %$self;
    $self -> { $_ } = $reset{ $_ } for keys %reset;
    return 0;
}