$number =~ /^(\D*\d){6}/g and $number =~ s/\G(\D*)\d(?=(?:\D*\d){4})/${1}X/g;

##</code><code>##

my ($s, $m ,$e) = $number =~ /^((?:\D*\d){6})(.*)((?:\d\D*){4})$/;
$m =~ tr/0-9/X/;
$number = "$s$m$e"

##</code><code>##

$number =~ s|^((?:\D*\d){6})(.*)(?=(?:\d\D*){4})|$1. ($2 =~ tr/0-9/X/r)|e;