I'm not sure what you want. Are you looking to be notifie when the child has been idle for a certain amount of time?

use strict;
use warnings;

use IO::Select qw( );

my $timeout    = 30.000;   # Notify user after idle for this many secs
my $command    = '...';    # The command to execute.
my $from_child = '';       # Receives the output of the command.

open(my $fh_from_child, "$command |")
   or die("Unable to launch $command: $!\n");

my $sel = IO::Select->new($fh_from_child);
for (;;) {
   my @r = $sel->can_read($timeout);
   if (!@r) {
      print("Another $timeout seconds has gone by...\n");
      redo;
   }

   read($fh_from_child, $from_child, 4096, length($from_child))
      or last;
}

print($from_child);
[download]

Tested on FreeBSD. It won't work on Windows (since you can't select pipes).