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();