Thanks for everyone's help:) Adding a read did the trick, and now $buf is filled with the stuff that I want!

use strict; 
use warnings;
use IO::Socket;

my $host = '199.82.244.14';
my $port = 13001;

my $data = '003 0000000073223                                     3000
+0073223091706407 20040513023';
my $msg_out = pack("nnna*", 0, 0, length $data, $data);
print "\$msg_out = -$msg_out-\n";

my $socket = IO::Socket::INET->new(    PeerAddr => $host,
                    PeerPort => $port
                    ) or die "Can't connect: $!\n";
print $socket $msg_out;
my $buf;
read($socket, $buf,6);
my ($tag,$code,$len) = unpack 'nnn', $buf;
print "\$tag = '$tag'\n";
print "\$code = '$code'\n";
print "\$len = '$len'\n";
read($socket, $buf, $len);
print "\$buf = '$buf'"; 

$socket->close or warn $@;
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