From @coke

There are a class of warnings that do not emit their warnings on
$*ERR, but always on the original stderr, which we can see by this
program which should eat both stdout and stderr, and does for most​:

$ cat warn.t
use MONKEY-SEE-NO-EVAL;
use IO​::String;

my $dummy-io = IO​::String.new();
sub MAIN($code) {
  $*OUT = $dummy-io;
  $*ERR = $dummy-io;
  try EVAL $code;
}

$ perl6 warn.t 'warn pi; say e'
$ perl6 warn.t "sink 3"
WARNINGS for /Users/wcoleda001/sandbox/perl6-doc/EVAL_0​:
Useless use of constant integer 3 in sink context (line 1)

$ perl6 warn.t 'my $a; my $a'
Potential difficulties​:
  Redeclaration of symbol '$a'
  at /Users/wcoleda001/sandbox/perl6-doc/EVAL_0​:1
  ------> my $a; my $a⏏<EOL>

This might be due to some code in src/Perl6/Actions that needs to be
updated to deal with the fact that the $*ERR and $*OUT might be
overridden.

--
Will "Coke" Coleda