ITYM that it includes the line number information, not $!. But that's because your string doesn't end with \n, it's ending in whatever sequence of escape characters RESET expands out to. You want to put the "\n\n" after the RESET.

. . . die RED, "Blah blah", RESET, "\n";
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Update: And you might want to test if STDERR is a tty or not before adding escapes to it in case the user is saving it into a file.

sub maybe_color {
  my $msg = shift;

  $msg = RED . $msg . RESET if -t STDERR;

  return $msg;
}
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