What I am doing wrong combining the two scripts?

use Tk;
my $mw = MainWindow->new();

{
    package Tie::Scalar::Callbacks;
    sub TIESCALAR {
        my $class = shift;
        my %self;
        @self{qw/ val store_cb fetch_cb /} = @_;
        return bless \%self, $class;
    }
    sub FETCH {
        my $self = shift;
        $self->{fetch_cb}->($self->{val}) if $self->{fetch_cb};
        return $self->{val};
    }
    sub STORE {
        my $self = shift;
        my $val = shift;
        $self->{store_cb}->($self->{val}, $val) if $self->{store_cb};
        $self->{val} = $val;
    };
    sub DESTROY { %{shift()}=() }
}


tie my $name, 'Tie::Scalar::Callbacks', "foo",
    sub { print "Entry value has changed!\n" };

my $label = $mw->Label(
    -text => 'Enter:',
)->grid(
    -row => 0,
    -column => 0,
);
my $entry = $mw->Entry(
    -textvariable => \$name
)->grid(
    -row => 0,
    -column => 1,
);

print "name is $name\n";
$name = "bar";
print "name is $name\n";
MainLoop;
[download]

(Note that I took away the button as I only a simulation of the third party component changing the scalar value)