#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

my $start_nonmask_nums = 6;
my $end_nonmask_nums = 4;


while (my $input = <>) {
    chomp $input;

    my $start_mask_pos = get_mask_pos($input, $start_nonmask_nums);
    my $start_string = substr $input, 0, $start_mask_pos;

    my $end_mask_pos = get_mask_pos(scalar reverse($input), $end_nonmask_nums);
    my $end_string = substr $input, -$end_mask_pos;

    my $mid_string = substr $input, $start_mask_pos, -$end_mask_pos;
    $mid_string =~ y{0-9}{X};

    my $masked_number = $start_string . $mid_string . $end_string;

    say $masked_number; 
}

sub get_mask_pos {
    my ($string, $count) = @_;

    $string =~ m{\d}g for (1 .. $count);

    return pos $string;
}

##</code><code>##

$ pm_number_mask.pl
541321069873210200
541321XXXXXXXX0200
5413 21069873210 200
5413 21XXXXXXXX0 200
54 13-21XX XXXXXX02#00
54 13-21XX XXXXXX02#00
1!2@3#4$5%6^7&8*9(0)-0_9+8=7{6}5[4]3<2>1
1!2@3#4$5%6^X&X*X(X)-X_X+X=X{X}X[4]3<2>1