If your code is more structured, it will be easier for other people to understand your code, and more monks will join to debug for you. More importantly, easier for yourself to debug.

Compare your code with the following rewrite (I am just trying to show a better structure, not doing everything), don't you think the rewrite is much easier to understand and debug?Maybe you can start from here.

use IO::Socket::INET;
use IO::Select;

use strict;
use warnings;

my $socket = new IO::Socket::INET(Proto => "udp");
my $select = new IO::Select($socket);        
my $num_packet_rcvd;
my $packet_rcvd;

for (4000 .. 4100) {#change this to your logic of going thru peer list
    my $peer = sockaddr_in($_, inet_aton("localhost"));
    $socket->send("1234567890", 0, $peer) || die "send: $!";#instead o
+f die, your real code shall handle it, for example logging
    print "packet sent to port $_\n";
}

while (1) {
    my @sockets = $select->can_read(10);
    last if ($#sockets == -1);#no more reply within a reasonable amoun
+t fo time
    $num_packet_rcvd ++;
    $socket->recv($packet_rcvd, 100);
    print "rcvd packet $num_packet_rcvd [$packet_rcvd]\n";#you will re
+place this with your stuff
}
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