After https://github.com/rakudo/rakudo/commit/704b893c6a the last tests fudged with this ticket are passing. I'm closing this ticket as 'resolved'.
[perl #126531] [JVM] throwing X::AdHoc "Cannot assign to a readonly variable or a value" instead of X::Parameter::RW
Christian Bartolomaeus via RT Sat, 26 May 2018 08:40:59 -0700