Another issue has just come to mind. With the object attributes being keyed on "$self", which includes the name of the class, you can no longer bless an object into a sibling class to implement state transitions.

Again, I guess some people would consider this a feature. I quite like the technique myself :-)

To overcome you'll need to strip out the class name. Assuming nobody changes the output format of references in later perl versions something like this should work.

package Foo;
use strict;
use warnings;
use overload;

my %foo = ();

my %self = ();    # we access self enough for it to be worth caching
sub self {
    my $self = overload::StrVal shift;
    return $self{$self} if exists $self{$self};
    $self{$self} = substr($self, index($self, '=')+1);
};

sub new {
    my $class = shift;
    bless [], $class;
};

sub foo {
    my $self = shift->self;
    $foo{$self} = @_ if @_;
    $foo{$self};
};

sub DESTROY {
    my $self = shift->self;
    delete $foo{$self};
    undef %self;
};
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Also, there is another advantage to Abigail-II's method. Storing the attributes in separate hashes should less expensive in memory - buckets cost less than hashes. Might make a difference if you're creating lots of objects.