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;