my $g = $public_key->granularity;
$public_key->granularity("*");
Granularity of the key. The value must match the Local-part of the effective "i=" tag of the DKIM-Signature header field. The granularity is a literal value, or a pattern with a single '*' wildcard character that matches zero or more characters.
If no granularity is defined, then the default value, '*', will be returned.