package OO::SubModule;

sub new {
  my ($class, $args) = @_;
  my $self = { ( defined $args ? %$args : () ) };
  bless($self, $class);
  
  ## Some boilerplate debug callback that we can override during construction
  $self->{debug_cb} //= sub { my $self = shift; carp("DEBUG ".join(", ", map { defined $_ ? $_ : "undef" } @_)); };
  
  return $self;
}

sub _debug {
  my $self = $_[0];
  goto &{$self->{debug_cb}};
}

## You can say $self->_debug("booya") to do $self->{debug_cb}->("booya") without it ending up in the call stack

##</code><code>##

## both apps

$oosub = OO::SubModule->new({
  debug_cb => sub {
    my $self = shift;
    my ($str) = @_;
    goto &{_debug};
  },
});

## Any occurrence of $self->_debug("booya") within OO::SubModule really does _debug("booya") of the main app, this way
## Without getting in the call stack, right?