[perl #131071] [BUG] unpacking a slurpy in `my` creates `Mu`s despite working when used in a routine's sig

[via RT]

# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #131071]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=131071 >



The last two items printed, `$type` and `$speedup`, get set properly if used in 
a sub's signature:

    `sub`
    14:34               m: sub ($sha, $, $, *@rest ($, $type, $speedup, $)) {dd 
[ @rest, $type, $speedup ];}(|'fa9aa47 [io grant] Make 
R::I::SET_LINE_ENDING_ON_HANDLE 4.1x Faster'.words)
    14:34       camelia         rakudo-moar 42f34f: OUTPUT: «[["Make", 
"R::I::SET_LINE_ENDING_ON_HANDLE", "4.1x", "Faster"], 
"R::I::SET_LINE_ENDING_ON_HANDLE", "4.1x"]␤»


But when using the same signature in a `my`, they are left as `Mu`s:
    `my`:
    14:34       Zoffix  m: my ($sha, $, $, *@rest ($, $type, $speedup, $)) := \ 
|'fa9aa47 [io grant] Make R::I::SET_LINE_ENDING_ON_HANDLE 4.1x Faster'.words; 
dd [ @rest, $type, $speedup ];
    14:34       camelia         rakudo-moar 42f34f: OUTPUT: «[["Make", 
"R::I::SET_LINE_ENDING_ON_HANDLE", "4.1x", "Faster"], Mu, Mu]␤»