my $ssh_b = Net::OpenSSH->new($host_b, ...);
$ssh_b->error and die;
my $tunnel_cmd = $ssh_b->make_remote_command({tunnel => 1}, $host_c, 22);
my $ssh_c = Net::OpenSSH->new($host_c, master_opts => [-o => "ProxyCommand=$tunnel_cmd"], ...);
$ssh_c->error and die;
$ssh_c->system('ls');