package SomeClass;
use Scalar::Util;

# The traditional id() function is modified: for a non-ref, return
# the argument, not undef
sub id { Scalar::Util::refaddr( $_[ 0]) || shift }

{
    # a field (attribute, object variable) with a read/write accessor
    my %some_field;
    sub some_field {
        my $obj = shift;
        $some_field{ id( $obj)} = shift if @_;
        $some_field{ id( $obj)};
    }

    sub init {
        my $obj = shift;
        $some_field{ id( $obj)} = shift;
        $obj;
    }

    sub new {
        my $class = shift;
        bless( do{ \ my $o})->init( @_);
    }
}

package main;

my $obj = SomeClass->new( 123);
print $obj->some_field, "\n";

SomeClass->init( 456);
print SomeClass->some_field, "\n";