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

[download]

or another somewhat less cryptic approach: 

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

[download]

or with a recent perl: 

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

[download]

In reply to Re: numbers masking by salva
in thread numbers masking by Anonymous Monk

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