variable expansion not working in $myauthservid definition?

[jasonsu at mail-central.com]

In amavisd.conf, if I set

	$mydomain = 'mail01.example.com';
	$myauthservid = 'amavisd.${$mydomain}';

received & scanned email's headers includes

	Authentication-Results: amavisd.${$mydomain} (amavisd-new);
		dkim=pass (2048-bit key) header.d=gmail.com

What's the correct usage so that $mydomain gets correctly expanded/used?

Usage of ${$mydomain} elsewhere in the config seems to work.  I'm missing something :-/