Oops, I guess I posted too early - there is a simple workaround:

my $ref;  # auxiliary to declaring @annotations

my @annotations = (
    [ "id",       [ ULONG, "value", undef ] ],
    [ "autoid",   $ref = [ ENUM, "AutoidKind", [ "SEQUENTIAL", "HASH" 
+] ],
                  [ $ref, "value", "HASH" ] ]
   );
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Sorry for the noise.