You really don't want to code this yourself. Here is a trivial example for interest sake only. It shows you how to build a packet (and its checksum) and that you do get data back which as you can see it is encoded in a binary packet format.

use IO::Socket;

use constant ICMP_ECHO      => 8;
use constant SUBCODE        => 0; # No ICMP subcode for ECHO and ECHOR
+EPLY
use constant ICMP_STRUCT    => "C2 n3 A64";

my $icmp = IO::Socket::INET->new(   PeerAddr => 'perlmonks.org',
                                    Proto=>'icmp'
);
print "Got socket\n";

my $data = '1'x64;
my $seq = 1;
my $checksum = 0; 
my $msg = pack(ICMP_STRUCT, ICMP_ECHO, SUBCODE, $checksum, $$, $seq, $
+data);
$checksum = checksum($msg);
my $msg = pack(ICMP_STRUCT, ICMP_ECHO, SUBCODE, $checksum, $$, $seq, $
+data);

print "Sending: $msg\n";
$icmp->send($msg);
$icmp->recv(my $buf, 1500);
print "Got: $buf";
print "Done\n";

sub checksum {
  my ($msg ) = @_;
  my $len_msg = length($msg);
  my $num_short = int($len_msg / 2);
  my $chk = 0;
  for my $short (unpack("n$num_short", $msg)) {
    $chk += $short;
  } 
  $chk += (unpack("C", substr($msg, $len_msg - 1, 1)) << 8) if $len_ms
+g % 2;
  $chk = ($chk >> 16) + ($chk & 0xffff);
  return(~(($chk >> 16) + $chk) & 0xffff);
}
[download]

I VERY MUCH APPRECIATE YOUR COMMENTS.

I can also be quite dense on some of these matters...but:
I am NOT trying to SEND an ICMP packet.
I AM trying to see if a remote UDP port is "open"
My assumption has been to send a UDP packet out
and if the port is CLOSED, then I get a ICMP error msg back
Otherwise I know the remote port is OPEN...

The code above sends an UDP packet out and an ICMP error msg is sent back. But I DON'T know how to grab it. I tried reading on the UDP and ICMP sockets but neither *seem* to have it.

Are you saying that I need to send an ICMP packet out???

Again, sorry if I am missing something completely obvious ;-)