#!/usr/bin/perl

use warnings;
use strict;

my $filename = "whateveryourfileiscalled.txt";
my $newfile = "whateveryouwantthechangedfiletobecalled.txt";


open (my $rfh,"<",$filename) or die "Can't open file $filename : $!";
open (my $wfh,">",$newfile) or die "Can't open file $newfile : $!";

while (my $line = <$rfh>) {
    if ($line =~ m/^HEADER/) {
        chomp $line;
        my $number = 42; # change to whatever number you want to use
        $line .= $number."\n";
    }

    if ($line =~ m/^REMARK/) {
        print {$wfh} "Extra line\n" # Change to whatever extra line you want
    }

    print {$wfh} $line;
}

close $rfh or die "Can't close $filename : $!";
close $wfh or die "Can't close $newfile : $!";