Using alarm to leave from `$cmd` prematurely leaves zombies as it jumps over the waitpid call inside that reaps the child process.

# untested!
my $pid = open my $out, '-|', $cmd or die "Open failed: $!";
local $INT{CHLD} = sub {};
my $fileno = fileno $out;
my $start = time;
my $str = '';
my $timedout = 0;
while (1) {
  my $v = '';
  vec($v, $fileno, 1) = 1;
  if (select($v, undef, undef, 1) > 0) {
    if (vec($v, $fileno, 1)) {
      sysread $v, $str, 1024, length $v or last;
    }
  }
  if (time - $start > $timeout) {
    $timedout = 1;
    kill INT => $pid 
}
close ($out);

print "timed out: $timedout\noutput: $str\n";
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