Preferably do not upvote it. (Downvote at your choice.)

Couldn't resist to upvote your post. Downvote mine if you like ;-)

I would change bless() to construct an inside-out object incorporating Alter into the core, so that bless { @_ }, $class provides a real inside-out object with a hidden alter ego for free, which is thread-safe, garbage-collected and accessible only inside this object's package:

package Foo;
sub new {
    my $class = shift;
    bless { @_ }, $class; # @_ goes into this ref's alter ego
}
sub bar {
    my $self = shift;
    $self->{bar} = shift if @_; # access $self's alter ego
    $self->{bar};
}

package main;

my $ref = Foo->new( bar => 'quux'); # empty blessed hashref returned
$ref->{bar} = 'blorf';              # used as a normal hashref

print $ref->bar, "\n";   # yields 'quux'
print $ref->{bar}, "\n"; # yields 'blorf'
[download]

That way, an object would behave as a normal perl data type if so treated, but as an object calling one of its methods. Private and public attributes for free.

No, I haven't thought about all the consequences...

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}