What you're looking for is a way to make perl more like C++ or Java, enforcing some sort of privilege (public, private, protected, package(Java)). Perl doesn't do that. Perl takes a completely different tract.

There is a simple convention in perl: any function with a leading underscore (which would be against The Rules(TM) in C - that is reserved for the compiler vendor) is considered "private." Other packages can call it, but it's not recommended - do so at your own risk.

That said, there are ways to enforce things like this. For example, you could create "sessions", and store the real object data in a private hash, e.g.:

my %data; # private hash
sub new
{
  my $data = rand();
  my $self = bless \$data;
  $data{$$self}{BKG_IMAGE} = 'image';
}

my $bkg_img = # private anon-sub
 sub {
  my $self = shift;
  if (@_) {
    $data{$$self}{BKG_IMAGE} = shift;
  }
  $data{$$self}{BKG_IMAGE};
}

# called as: $self->$bkg_img('new_image')
# or         $image = $self->$bkg_img();
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Not recommended. Just document that _bkg_img is a private internal method. You'd be surprised how often a method you think should be private really shouldn't be ;-)