eval {
        local $SIG{ALRM} = sub { die "tcpdump Died\n" };
        alarm 20;
        @captured_data = `$tcpdump -i $nic -c1 -xx -s0 'ether[20:2] == 0x2000' 2>/dev/null`;
        alarm 0;
    };

    # Check the status of eval to see if we alarmed out
    if ($@) {
        return;
    } else {
        return @captured_data;
    };
};

After the script runs:
grapeape:~# ps -ef |grep tcpdump |grep -v grep
root      3499     1  1 16:26 pts/1    00:00:00 sh -c /usr/sbin/tcpdump -i eth0 -c1 -xx -s0 'ether[20:2] == 0x2000' 2>/dev/null
root      3500  3499  2 16:26 pts/1    00:00:00 /usr/sbin/tcpdump -i eth0 -c1 -xx -s0 ether[20:2] == 0x2000