From @briandfoy

Here's a program that starts another program with run() with
various combinations of :out and :err. When :out and :err are both
specified, the exit code is incorrect. This is 2017.01​:

  #!/Applications/Rakudo/bin/perl6

  use v6;

  my $filename = 'null-program.p6';
  my $fh = open $filename, :w;
  END { unlink $filename }

  $fh.put( q​:to/HERE/ );
  #!{ $*EXECUTABLE }
  $*ERR.say( Q/This goes to standard error/ );
  $*OUT.say( Q/This goes to standard output/ );

  exit 137;
  HERE

  {
  my $proc = run $*EXECUTABLE, $filename;
  say "1. Exit is { $proc.exitcode }";
  }

  {
  my $proc = run $*EXECUTABLE, $filename, :out;
  $proc.out.slurp-rest( :close );
  say "2. Exit is { $proc.exitcode }";
  }

  {
  my $proc = run $*EXECUTABLE, $filename, :err;
  $proc.err.slurp-rest( :close );
  say "3. EHere's a program that starts another program with run() with
various combinations of :out and :err. When :out and :err are both
specified, the exit code is incorrect​:

  #!/Applications/Rakudo/bin/perl6

  use v6;

  my $filename = 'null-program.p6';
  my $fh = open $filename, :w;
  END { unlink $filename }

  $fh.put( q​:to/HERE/ );
  #!{ $*EXECUTABLE }
  $*ERR.say( Q/This goes to standard error/ );
  $*OUT.say( Q/This goes to standard output/ );

  exit 137;
  HERE

  {
  my $proc = run $*EXECUTABLE, $filename;
  say "1. Exit is { $proc.exitcode }";
  }

  {
  my $proc = run $*EXECUTABLE, $filename, :out;
  $proc.out.slurp-rest( :close );
  say "2. Exit is { $proc.exitcode }";
  }

  {
  my $proc = run $*EXECUTABLE, $filename, :err;
  $proc.err.slurp-rest( :close );
  say "3. Exit is { $proc.exitcode }";
  }

  {
  my $proc = run $*EXECUTABLE, $filename, :out, :err;
  $proc.out.slurp-rest( :close );
  $proc.err.slurp-rest( :close );
  say "4. Exit is { $proc.exitcode }";
  }

The fourth situation reports a different exit code​:

  This goes to standard error
  This goes to standard output
  1. Exit is 137
  This goes to standard error
  2. Exit is 137
  This goes to standard output
  3. Exit is 137
  4. Exit is 0xit is { $proc.exitcode }";
  }

  {
  my $proc = run $*EXECUTABLE, $filename, :out, :err;
  $proc.out.slurp-rest( :close );
  $proc.err.slurp-rest( :close );
  say "4. Exit is { $proc.exitcode }";
  }

The fourth situation reports a different exit code​:

  This goes to standard error
  This goes to standard output
  1. Exit is 137
  This goes to standard error
  2. Exit is 137
  This goes to standard output
  3. Exit is 137
  4. Exit is 0

Code snippet that is not mangled:

use v6;

my $filename = 'sandbox/null-program.p6';
my $fh = open $filename, :w;
END { unlink $filename }

$fh.put: ｢
$*ERR.say( Q/This goes to standard error/ );
$*OUT.say( Q/This goes to standard output/ );

exit 137;
｣;

{
my $proc = run $*EXECUTABLE, $filename;
say "1. Exit is { $proc.exitcode }";
}

{
my $proc = run $*EXECUTABLE, $filename, :out;
$proc.out.slurp-rest( :close );
say "2. Exit is { $proc.exitcode }";
}

{
my $proc = run $*EXECUTABLE, $filename, :err;
$proc.err.slurp-rest( :close );
say "3. Exit is { $proc.exitcode }";
}

{
my $proc = run $*EXECUTABLE, $filename, :out, :err;
$proc.out.slurp-rest( :close );
$proc.err.slurp-rest( :close );
say "4. Exit is { $proc.exitcode }";
}

Output on all releases: https://gist.github.com/AlexDaniel/fca7b261d6a4e46290d2fdb7645e5e2a

Seems to be fixed after (2017-06-12) rakudo/rakudo@92bd7e4 just like #4456