Me hearties, it be a fine piece o' code. But ye canna write:

my $grammar = q {
  startrule: test
    test:      "primary"(?) "key"
};
[download]

Cause ye gets an error! Arr!

ERROR (line 4): Untranslatable item encountered: "?"
                (Hint: Did you misspell "?" or forget to comment it ou
+t?)
[download]

RD_TRACE told me:

Parse::RecDescent: Treating "startrule:" as a rule declaration
    Parse::RecDescent: Treating "test" as a subrule match
    Parse::RecDescent: Treating "test:" as a rule declaration
    Parse::RecDescent: Treating ""primary"" as an interpolated literal
                       terminal
    Parse::RecDescent: Treating "( ? )" as an implicit subrule
    Parse::RecDescent: Treating "_alternation_1_of_production_1_of_rul
+e_test
                       :" as a rule declaration

       ERROR (line 4): Untranslatable item encountered: "?"
                (Hint: Did you misspell "?" or forget to comment it ou
+t?)
    Parse::RecDescent: Treating "_alternation_1_of_production_1_of_rul
+e_test"
                       as a subrule match
    Parse::RecDescent: Treating ""key"" as an interpolated literal ter
+minal
[download]

Arr.