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

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

    print {$wfh} $line, "\n";
}