Additional checkout block fields – validation

Hello, you provide an example of validation can also be against other fields in your documentation:

https://developer.woocommerce.com/docs/block-development/cart-and-checkout-blocks/additional-checkout-fields/#conditional-visibility-and-validation-via-json-schema

woocommerce_register_additional_checkout_field(
		array(
			'id'                => 'plugin-namespace/alt-email',
			'label'             => 'Confirm our email address',
			'location'          => 'contact',
			'type'              => 'text',
			'required'          => true,
			'attributes'    => array(
					'autocomplete'     => 'off',
			),
			'sanitize_callback' => function ( $field_value ) {
				return sanitize_email( $field_value );
			},
			'validation' => array(
				"type" => "string",
				"format" => "email",
				"not" => array(
					"const" => array(
						"$data", "0/customer/billing_address/email"
					)
				),
				"errorMessage" => "Please ensure your email address matches the confirmation.",
			),
		)
	);

Variable $data should to refer to the current field value, but it makes an error. Please help.