[perl #131251] Useless use warning triggered on assignment forms of reverse metaops

[Brian S. Julin]

# New Ticket Created by  "Brian S. Julin" 
# Please include the string:  [perl #131251]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=131251 >



An R metaop using the bracket notation to retain lvalueness of the lvalue side
triggers a "useless use" warning in sink context, and shouldn't.

(10:00:22 PM) skids: m: my $a = "H"; say $a [R~]= "T"; $a.say; $a [R~]= "W"; 
$a.say
(10:00:22 PM) camelia: rakudo-moar f4cbdb: OUTPUT: «Potential difficulties:␤    
Useless use of [R~]= in sink context␤    at <tmp>:1␤    ------>  $a = "H"; say 
$a [R~]= "T"; $a.say; $a ⏏[R~]= "W"; $a.say␤TH␤TH␤WTH␤»