# untested!
use POSIX qw(WNOHANG _exit);
use Errno qw(ECHILD);

local $SIG{CHLD} = sub {}; # this do-nothing sub ends the
                           # select below on SIGCHLD

my $pid = fork;
if (!$pid) {
    defined $pid or die "fork failed: $!";
    do { exec @cmd }; # warnings friendly!
    _exit(1);
}

my $start = time;
while (1) {
  my $rc = waitpid ($pid, WNOHANG);
  last if $rc == $pid;
  last if $rc < 0 and $! == ECHILD;
  select (undef, undef, undef, 1);
  kill INT => $pid if time > $start + $timeout;
}