I don't need to return a constant under any circumstance, but if I know I am returning an rvalue, I can return: 
p->{_path}

[download]
If something is being assigned to 'path', then I'd know I need the lvalue -- and return something like 
$p->{tied_path}

sub callback_for_STORE_into_tied_path { my $p=shift;
  my $newvalue;
  $p->{_path}=$newvalue;
  $p->{_url}=undef;
  ...
  $p->{_path};
}
...later...in 
sub url(;$) { my $p=shift;
  $p->compose_url_from_parts() unless defined $p->{_url};
  $p->{_url}
}

[download]
So if someone want to read path, they would get a simple, direct reference (no tied var overhead), but if they wrote to it, it needs to store the new value, and clear the internal value for _url so it can be re-composed.

At no point should I need to return a constant -- just a choice between the actual var in the hash, or a tied var.

If I knew the lvalue-context, is there a reason why that wouldn't work?

In reply to Re^8: can sub check context for lvalue vs rvalue context? by perl-diddler
in thread can sub check context for lvalue vs rvalue context? by perl-diddler

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