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my $str = '4512872356698';
$str =~ s/^\d{6}\K\d+(?=\d{4})/'X' x ($+[0] - $-[0])/e;
print "$str\n";

my $str = '4512872356698';
substr($str, 6, -4, 'X' x (length($str) - 10));
print "$str\n";