$str = $a . $b . $c .$d;
$xpos_spot = length( $str );
$e = '0000000000';
$xpos_len = length($e);
$str .= $e;
$str .= $f;
...
$str .= $w;
substr $str, $xpos_spot, $xpos_len, 
        sprintf('%${xpos_len}d',length($str));
$str .= $x;
...