How about 

use strict;
use warnings;

my $str = '4512872356698';
my $len = length $str;
substr($str, 6, $len - 10) = 'X' x ($len - 10);
print "$str\n";

[download]

Using lvalue substr for good effect.

Perl 6 - second systems done right

In reply to Re: regular expression by moritz
in thread regular expression by Anonymous Monk

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