#!/usr/bin/perl

use warnings;
use strict;

use File::Find;

open my $logfilefsv, '>>', 'logfsv.txt' or die $!;

my $file = "Mark.txt";

open my $in_fh, '<', $file or die $!;
my @lines = <$in_fh>;
close $in_fh;

open my $out_fh, '>', $file or die $!;

print $logfilefsv @lines;

for my $line ( @lines ){
    $line =~ s/N/M/gi;
    print $out_fh $line;
}