Do you want to kill the child if it's inactive for 5s or if it's not done in 5s? The former is quite simple (alarm around a read), but it looks like you want the latter. 

use Time::HiRes qw( time );
use IO::Select  qw( );

...

my $buf = '';
if (!eval {
   my $pid = open(my $fh, '-|', @cmd)
      or die("Can't create child: $!\n");

   my $deadline = time() + 5;
   my $sel = IO::Select->new($fh);
   for (;;) {
      my $time_left = $deadline - time;
      die("Timeout\n") if $time_left <= 0;

      next if !$sel->can_read($time_left);

      my $rv = sysread($fh, $buf, 64*1024, length($buf);
      die("Can't read from child: $!\n") if !defined($rv);
      last if !$rv;
   }

   waitpid($pid, 0);
   die("Can't waitpid: $!\n")                      if $? == -1;
   die("Child killed by signal ".($? & 0x7F)."\n") if $? & 0x7F;
   die("Child returned error ".($? >> 8)."\n")     if $? >> 8;

   1
}) {
   my $e = $@;
   kill(TERM => $pid);
   waitpid($pid, 0);
   die($e);
}

...

[download]

Not tested.

Update: Added missing my $pid =.

In reply to Re: fork - alarm - output by ikegami
in thread fork - alarm - output by chart3399

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