In Apocalypse 5, Larry uses ::, not . to separate grammar and rule names.
$text =~ /<Diff.file>/; # Invoke through grammar
This is a method call.
Update: As Juerd rightly points out, there is a distinction between a grammer and a class, and their rules and methods. However, the same syntax is used to call a rule from a grammer, as a grammer can inherit from other grammer(s), delegate to other grammers, and possibly even autogenerate rules. In these cases, the rule needs to be treated as a method call.
Light a man a fire, he's warm for a day. Catch a man on fire, and he's warm for the rest of his life. - Terry Pratchet