#!/usr/bin/perl

open (logfilefsv, '>>logfsv.txt');

$file = "Mark.txt";

open(IN, "+<", $file);
#my $old = "N";
#my $new = "M";
@lines = <IN>;

print logfilefsv @lines;

seek IN, 0, 0; # seek to top of file
truncate(IN, 0); # truncate old data

foreach $line (@lines) {
       $line =~ s/N/M/gi;
       print IN $line;
       }
__END__


##</code><code>##

#!/usr/bin/perl

{
local ($^I, @ARGV) = ('.bak', shift );
 while (<>) {
    $_ =~ s/M/N/gi; 
    print "$_";
 }
}
__END__