I'm no networking guru but here's some untested code that just might do the required input processing reliably.

# $sock is open, probably is a IO::Socket of some type

my $total_read = 0;
my $full_data = '';
my $total;

while ( 1 ) {
    my $read = 0;
    while ($read < 4) {
        $read += read($sock,$data,4-$read,$read)
        # not sure if last arg should have a +1 on it
    }
    $total_read += $read;
    
    if (! $total) {
        $total = unpack("N",$data);
    }
    elsif ( $total_read >= $total + 4 ) {
        process(substr($data,0,$total_read-4-$total));
        $full_data .= substr($data,0,$total_read-4-$total);
        last;
    }
    else {
        $full_data .= $data;
        process($data);
        other_process($full_data);
    }
}

full_process(substr($full_data,0,$total));
[download]

And the sender is

# $data has the required data and $sock is an open socket
print $sock pack('N',length($data)) . $data . ("\0" x (4-(length($data
+)%4)));
[download]