Hello I changed my program . And added the features you suggested. I know a better way must exist . Can any one help.

$openx = open(BAKFIL, "<data.txt"); 

open(BAKFIL2, ">data.bak.txt"); 

   if ( ! $openx ) { 

     print "Content-type:text/html\n\n"; 
     print "File Does not exist. Program will now abort";
close( BAKFIL );
close( BAKFIL2 );
unlink( "data.bak.txt");
exit ; 

}
  
    else {
      while(<BAKFIL>) {
      print BAKFIL2 $_;
}

close( BAKFIL );
close( BAKFIL2 );

}

my $filesize = -s "data.txt";


open(XFIL,"data.txt") || die $!;
        @main = <XFIL>;
        close(XFIL);


open(XFIL,">data.txt") || die $!;
 foreach $main_line (@main) {
          

   print XFIL $main_line;     

 if ($main_line =~ /The marker line/) {

 print XFIL "Hello\n";


}

}

close(XFIL);

my $filesizemain = -s "data.txt";

if  ($filesizemain < $filesize) {
open(BAKFIL2, "<data.bak.txt") || die $!; 

open(BAKFIL, ">data1313.txt") || die $!; 

   while(<BAKFIL2>) {
      print BAKFIL $_;
}

close( BAKFIL2 );
close( BAKFIL );
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