why i don't receive DHCPDiscover message from server using dongle and why same code give DHCPNac on wifi

I have written a script using perl and i have tested it using two different ways: (1)Using a wifi connection (I have router which is directly connected to server and i am connected to that router). When i run my script on wifi connection, I receive DHCPOffer with IP and when i send request than i receive DHCPNack on that Request. i don't know why ? (2) Using the same code when i connect using a dongle then i even don't receive DHCPDiscover it just kind of hang for about hours and there is no any kind of message from server.

Below is all my code :

use IO::Socket::INET;
use Net::DHCP::Packet;
use Net::DHCP::Constants;
my $result='false';
my $nowtime = localtime(time);
print "\nprint $nowtime\n";
SocketCreation();
if ( $result eq 'true')
{
print "\nTRUE\n";
}
else
{
print "\nFALSE \n";
}
#########Here is the finction definition to SocketCreation ###########
+################
sub SocketCreation()
{
my $br_addr = sockaddr_in( '67', inet_aton('255.255.255.255') );
my $xid     = int( rand(0xFFFFFFFF) );
#my $chaddr  = '00189BF9C1Dp';
my $chaddr = '0016cbb7c882';

my $socket = IO::Socket::INET->new(
    Proto     => 'udp',
    Broadcast => 1,
    LocalPort => '68',
) or die "Can't create socket: $@\n";
print "\n please wait and have patience......:\n";
my $discover_packet = Net::DHCP::Packet->new(
    Xid                           => $xid,
    Chaddr                        => $chaddr,
    Flags                         => 0x8000,
    DHO_DHCP_MESSAGE_TYPE()       => DHCPDISCOVER(),
    DHO_HOST_NAME()               => 'Perrr',
    DHO_VENDOR_CLASS_IDENTIFIER() => 'perlr',
);
 print "/n just a check\n";
$socket->send( $discover_packet->serialize(), 0, $br_addr)
    or die "Error sending:$!\n";

my $buf = '';
print "/n just a check2\n";
$socket->recv( $buf, 1024) or die "recvfrom() failed:$!";
#print STDERR $socket->toString();
my $resp = new Net::DHCP::Packet($buf);
print "Details:\n" . $resp->toString();
$ip_offerto=$resp->yiaddr();
print "\n IP we got from google is : $ip_offerto\n";
$result = ($ip_offerto =~ /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/) ? "t
+rue":"false";
print $result;
# create DHCP Packet REQUEST
$request = Net::DHCP::Packet->new(
                      Xid => 0x12345678,
                      Ciaddr => $resp->yiaddr(),
                      DHO_DHCP_MESSAGE_TYPE() => DHCPREQUEST(),
                      DHO_VENDOR_CLASS_IDENTIFIER() => 'ShekharsIPRequ
+est',
                      DHO_DHCP_REQUESTED_ADDRESS() => $resp->yiaddr(),
                      );
print"\nSending REQUEST to server\n";
print $request->toString();
$socket->send($request->serialize(),0,$br_addr)
              or die "Error sending:$!\n";

print "\nWaiting for response from server\n";
$socket->recv($buf, 4096) || die("recv:$!");

print "\nGot response\n";
$resp = new Net::DHCP::Packet($buf);
print STDERR $resp->toString();
close($socket);
}

Actually could some one please give me solution for :

(1) Why it give DHPCNACK when i use WIfi (through another roter) ?
 (2) Why the same code hangs while run through dongle (USB Modem internet device)?