{
   package AtomicState;  # yucky name!

   sub new {
      my ($class) = @_;
      return bless({}, $class);
   }

   sub transaction {
      my ($self) = @_;
      return AtomicState::Transaction->new($self);
   }
}

{
   package AtomicState::Transaction;

   sub new {
      my ($class) = @_;
      my $obj_ref = \$_[1];
      my $self = bless([ $obj_ref, undef ], $class);
      $self->commit();
      return $self;
   }

   sub start_transaction {
      my ($self) = @_;
      my $backup = freeze($self);
      return AtomicState::Transaction->new($self);
   }

   # Multiple commits are allowed.
   sub commit {
      my ($self) = @_;
      $self->[1] = freeze(${$self->[0]});
   }

   # Rolls back to the last commit, the first
   # being the creation of the the Transaction.
   sub rollback {
      my ($self) = @_;
      ${$self->[0]} = thaw($self->[1]);
   }

   sub DESTROY {
      my ($self) = @_;
      $self->rollback();
   }
}


my $state = AtomicState->new();

{
   my $transact = $state->transaction();

   $state->{a} = ...;
   push @{$state->{b}}, ...;
   $state{c}->{key} = ...;

   if (success) {
      $transact->commit();
   }
}