package My::A;
...
package My::A::_B;
    sub TIESCALAR { return bless \(my $foo = $_[1]), $_[0] }
    sub FETCH { ${ $_[0] } }
    sub STORE { ${ $_[0] } = $_[1] }
package My::A;

sub method {
    my ($self, @foo) = @_;
    tie my $bar, 'My::A::_B', $foo[0];
    ...
}
...