use warnings;
use strict;


package MyThing;

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

sub set_foo {
  my ($self, $newfoo) = @_;
  $self->[0] = $newfoo;
}

sub get_foo {
  $_[0]->[0];
}


package HashAPI;

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

sub STORE {
  my ($self, $key, $val) = @_;
  unless ($key eq 'foo') {
    die "No $key member\n";
  }
  $self->[0] = $val;
}

sub FETCH {
  my ($self, $key, $val) = @_;
  unless ($key eq 'foo') {
    die "No $key member\n";
  }
  $self->[0];  
}

sub new {
  my $class = shift;
  tie my(%obj), $class;
  bless \%obj, $class;
}


package main;

my $clunker = MyThing->new();
my $slick = HashAPI->new();

$clunker->set_foo('Some value');
print $clunker->get_foo, "\n";

$slick->{foo} = 'Some value';
print $slick->{foo}, "\n";