[perl #130441] [REGRESSION] Writablity of elements of slurpy params depends on given arguments

[Zoffix Znet via RT]

This seems very related:

  <Zoffix_> m: sub foo (*@foo) { dd @foo[1] = 42 }; foo <foo bar>; foo |<foo 
bar>;
  <camelia> rakudo-moar 88140b: OUTPUT: «42␤Cannot modify an immutable Str␤  in 
sub foo at <tmp> line 1␤  in block <unit> at <tmp> line 1␤␤»

It actually affects whether values in a created Map are mutable, *depending on 
how the Map was created*:

  <Zoffix_> m: dd Map.new(<foo bar>)<foo> = 42
  <camelia> rakudo-moar 88140b: OUTPUT: «Int <element> = 42␤»
  <Zoffix_> m: dd Map.new(|<foo bar>)<foo> = 42
  <camelia> rakudo-moar 88140b: OUTPUT: «Cannot modify an immutable Str␤  in 
block <unit> at <tmp> line 1␤␤»