Files can only contain bytes, not keystrokes. So, in addition to finding a method of obtaining the keystrokes (Term::ReadKey?), you need to decide how you want to store them.

Update: I interpreted your question rather literally, but you didn't give us much to go on.

some work around using Term::Readkey

It may help for your requirement

#! /usr/bin/perl
use Term::ReadKey;
ReadMode 4; # Turn off controls keys

open(FH,'>',"file");
$|=1;
## To read 10 characters and write into the file

$i=0;
## for reading 10 characters....
while ( $i<10)
{
    until (defined ($key = ReadKey(-1))) {}
    print FH "$key\n";
    $i++;

}
close FH;
ReadMode 0; # Reset tty mode before exiting
[download]

Thank you very much! But what I am looking for is, when this script is running, any keys strokes made in other applications too, for example, I have started this script and type something in notepad, those keys should also be recorded into this file.

The problem is what do you mean by user? A user typing at a single terninal? All keystrokes any user enters? If that is the case, you will be better off with a kernel patch.....google for "linux kernel patch keystroke recorder".

.oO(Keylogger?)