I know, I told you what was wrong and what had to be fixed to get your code to overwrite itself with "hello!", which is what you seem to want to do for some reason.

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

Should be:

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

Had you read any of the links I provided you'd be using something like:

use strict;
use warnings;
use diagnostics;

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

Wow! when i run the following script then hello! can be seen inside "not.txt" file after once scrip is run

use strict;
use warnings;
use diagnostics;

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

it means book assumed me as knower of perl so it didn't mentioned to write

use strict;
use warnings;
use diagnostics;
[download]

before that script.anyway in the book above example the line is" Here's an example in which we open a file for output and print some text to that file". So i think it just replaced(or write) that whole script into word "hello"! and if i write different word then that word will be there after scrip running.

"Wow! when i run the following script then hello! can be seen inside "not.txt" file after once scrip is run."

I've already questioned why you'd want to do this, you didn't answer.

"it means book assumed me as knower of perl so it didn't mentioned to write:..."

I have no idea what book you're talking about. For all I know this is explained in a previous section of the text. It's also possible that you didn't follow the books written instructions properly. I don't believe the book told you to save the perl script as not.txt, which the example script overwrites.