Sounds a bit like you're trying to break encapsulation - which is probably a bad thing.

Given that all access to the values in $self will be via accessor functions (or, at least, they should be) why not let the accessor functions deal with keeping the two values in step.

sub some_key_from_tn {
  my $self = shift;

  if (@_) { # called to set value
    $self->{some_key_from_tn} = $_[0];
    # call Telephone::Number method to set value
    $self->{tn}->some_key_from_tn($_[0]);
  }

  $self->{some_key_from_tn};
}
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