nohup only does its redirection magic when stdin/stdout/stderr are connected to a tty, that's why you need to explicitly redirect these streams.

Besides that, you can use Net::OpenSSH spawn method to launch the command in the background:

# this code does not check for errors!

my $pid = $ssh->spawn('/usr/sbin/tcpdump -i eth1 -s 1500 -w /tmp/tmp.p
+cap');
($out,$err) = $ssh->capture2('ping -I eth1 -c 3 192.168.107.254');

$ssh->system('killall tcpdump');
waitpid($pid, 0);

($out,$err) = $ssh->capture2('tcpdump -r /tmp/tmp.pcap');
print "THE OUT IS $out\nTHE ERROR IS $err\n";
[download]