I'm not quite sure I understand your question. Perhaps you want:

sub not_exposed_reason {
    my ($self, $not_exposed_reason) = @_;
    $self->{_not_exposed_reason} = $not_exposed_reason // "No exposed 
+reason";
}
[download]

On older Perls, you may want to write that as:

sub not_exposed_reason {
    my ($self, $not_exposed_reason) = @_;
    $self->{_not_exposed_reason} = defined $not_exposed_reason ? $not_
+exposed_reason : "No exposed reason";
}
[download]