Here you mention you have "some chapters" from this book, and that some of the print is bad. You've been given links several times now to up to date Perl documentation as well as tutorials and other resources for learning Perl. IMHO this is a better starting than some extracts of dubious (print) quality.

I have created following code to display hello! on command line screen

use strict;
use warnings;
use diagnostics;

open (DOG, ">not.txt") or die ("Cannot open not.txt");
print DOG "hello!";
open (DOG, "<not.txt") or die ("Cannot open not.txt");
while(<DOG>){
print;}
close (DOG);
[download]

And with previous code

use strict;
use warnings;
use diagnostics;

open (DOG, ">not.txt") or die ("Cannot open not.txt");
print DOG "hello!";
close (DOG);
[download]

I was thinking that the code only creates file and doesnot display contents of file on the command line screen.

I'm not sure what you mean. I'm sure you can run your code and understand the input and output. If not you've been given links to places which explain how all this works.