How do I make this possible?

Example:

#!/usr/bin/perl
use strict; use warnings;
use IO::Select;

my ($rh,$wh);

pipe($rh,$wh);
$wh->autoflush;


if ((my $pid = fork) == 0) {
    # child here
    close $rh;
    sleep 2;
    print $wh "child here\n";
    sleep 4;
    print $wh "child again\n";
    close $wh;
    exit;
} else {
    # parent here
    close $wh;
    my $s = IO::Select->new();
    $s->add($rh);
    $SIG{ALRM} = sub {
        if ($s->can_read(0) and ! eof($rh) ) {
            my $line = <$rh>;
            print "child sent: $line";
        }
        alarm 1;
    };
    alarm 1;
    for(;;) { # body of your parent code
        print "waiting...\n";
        sleep;
        print "again ";
        last if eof $rh;
    }
    print "child is done\n";
}
[download]

For something more elaborate, check out an event module, e.g. EV and use EV::io with $rh. If you already use some library using an event loop (e.g. Tk), use that.