in reply to Parent process finished. How to exit the child process.

Just use ReadKey in timed mode: 

#! perl -slw
use strict;
use Term::ReadKey;

ReadMode 4;

my( $count, $c );
until( defined( $c = ReadKey( 1 ) ) ) {
    print ++$count;
}
ReadMode 0;

[download]
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
RIP an inspiration; A true Folk's Guy

In Section Seekers of Perl Wisdom