See Is there any way to record keystrokes. If you want to make a stealth keylogger Perl is not the correct choice of language, nor is this an appropriate forum.

I am almost certain this is not what you want, but it is all I am going to give you. Building stealth keyloggers is not in my list of useful tasks.

use Term::ReadKey;
while (defined ($key = ReadKey(0)) ) {
    open F, '>>./keys.dat' or die $!;
    print F $key;
    close F;
}
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