perl6

pmichaud@plum:~/rakudo$ ./perl6
> my $b = 5; sub foo($a) { $b++; $a++; say $a; };  foo($b)
Cannot assign to readonly variable.
in sub foo (<unknown>:1)
called from Main (<unknown>:1)
> 

perl6

$ cat z
grammar ABC {
    token TOP { abc | <oops: "Failed to find abc!"> }
    method oops($msg) { die $msg; }
}

ABC.parse('xyz');

$ ./perl6 z
Failed to find abc!