If that's what you want, I'd recommend using fork. I did something similar a while back, using fork, as described in Timing and timing out Unix commands. If you get it working without using fork, I'd be interested in seeing your code.

I had a similar requirement, and came up with this:

my %p;

sub killchildren {

    my $ppid = shift;

    chomp(my $out = `ps -o pid --no-headers --ppid $ppid`);

    for (split /\n/, $out) {
        killchildren($_);
        kill 9, $_;
    }
}

sub homicide {

    killchildren($$);

    $p{fg}{timeout} = 1;
}

sub fg {

    my ($cmd, $timeout) = @_;

    $p{fg}{timeout} = 0;

    my $out;

    eval {
        local $SIG{ALRM} = q(homicide);
        alarm $timeout;
        $out = `$cmd`;
        alarm 0;
    };

    return $out unless $p{fg}{timeout};

    return q(TIMEOUT);
}

#
# TEST
#
my $out = fg("echo Hi; sleep 5", 1);
print "Out = $out\n";

$out = fg("echo Bye; sleep 5", 10);
print "Out = $out\n";
[download]

The test example had the following output:

~/src$ ./a.pl
Out = TIMEOUT
Out = Bye