(talking unix/linux) If you want to be able to control a child process, you need it's PID. You get it either with fork:

my $pid;
if (($pid = fork()) == 0) {
    # child process
    exec 'some/program','and', 'args';
} else {
    # parent process
    $SIG{'CHLD'} = \&sub_that_handles_child_exit_notify;
}
# dosomething with $pid...
my $is_alive = kill 0, $pid;  # see if still alive
kill 15, $pid;                # send a SIGTERM
[download]

my $pid = open(FH,"process @args |"); # to read from it
my $pid = open(FH,"| process @args"); # to write to it
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For handling input and output, see IPC::Open2. For handling STDERR as well, IPC::Open3. To set timeouts, see alarm.

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}