##
11 12 13 14 15
...478901234567890123456789012345678901234567890
##
##
sub ruler {
my($n,$r) = ($_[0], ' ');
$r .= ' 'x(10-length).$_ for (1..($n+9)/10);
return "$r\n" . '0123456789' x (($n+9)/10) . "0\n";
}