[perl #130611] hash slice assignment doesn't work for custom class

[via RT]

# New Ticket Created by  Wenzel Peppmeyer 
# Please include the string:  [perl #130611]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=130611 >


class C does Associative {
     method AT-KEY(\k){ dd 'at-key'; True };
     method ASSIGN-KEY(\k, \v){ dd 'assign-key'~k~v }
}
C.new<a> = 1;
C.new<a b> = 1,2;
# OUTPUT«"assign-keya1"␤"at-key"␤"at-key"␤Cannot modify an immutable Bool␤  in 
block <unit> at <tmp> line 1␤␤»

# ASSIGN-KEY is never called for list keys as a result a custom class
# (that is not a container itself) can not provide hash slices without
# significiant hackery.