{
  # these are only visible to the function
  my @settings = (
    [ (0,0), "red", "E = 0" ],
    [ (1e-99, 1e-90), "orange", "1e-99 < E < 1e-90" ],
    ...
  );

  sub color_and_msg {
    my ($boo, $tab, $balloon) = @_;
    my ($color, $msg) = ("black", "");

    for (@settings) {
      my ($low, $high, $c, $m) = @$_;
      if ($low <= $boo and $boo < $high) {
        ($color, $msg) = ($c, $m);
        last;
      }
    }

    my $frame = $tab->Label(
      -bg => $color,
      -relief => 'sunken',
      -width => 10,
    );

    $balloon->attach($frame, -balloonmsg => $msg)
      if $msg;

    return $frame;  # if you need it
  }
}

##</code><code>##

my $frame = color_and_msg($boo, $tab, $balloon);