[perl #125590] LTA error message when you try to modify an ro parameter

[via RT]

# New Ticket Created by  Rob Hoelz 
# Please include the string:  [perl #125590]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=125590 >


Simliar to https://rt.perl.org/Ticket/Display.html?id=125531, but that ticket 
appears for more subtle reasons ($/ being modified as the result of operations 
like ~~).

If I try to modify an ro parameter, or the parameter to a for loop (which, when 
using ->, I guess is another sort of ro parameter), I get one of the two error 
messages:

    sub foo(Str $str) {
        $str ~= 'another';    # Cannot assign to an immutable value
        $str .= substr(0, 3); # Cannot modify an immutable Str
    }

I've also seen "Cannot assign to a readonly variable or a value".

While the error message makes sense, it may be nice to...

  - Indicate the variable being assigned to, if it is indeed a variable.
  - (In the case of the linked ticket mentioning $/) Indicate the operation 
responsible for the writing
  - (Other than for $/) Suggest is copy or <-> if that is what the user means.