If you are using MS-Windows, you probably need to do "binmode" on both the input and output files after you open them, to turn off "crlf" mode. If you don't do that, the binary data will get corrupted.

Apart from that, you really should set the input recorder separator variable to "AZII*" (as recommended in replies to your previous thread), because using line-oriented i/o on non-text data is just strange. Try something like this:

use strict;

open( FILE, '1.dat') or die "1.dat: $!";
binmode FILE;
$/ = "AZII*";

my $num = 0;
while (<FILE>) {
    chomp;
    next unless length();  # skip the initial (empty) input record

    $num++;
    open (DES, ">out/$num.txt") or die ( "out/$num.tiff" );
    binmode DES;
    print DES "AZII*".$_;
    close DES;

    last if ( $num == 5 );
}
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(Updated to add the "next unless length()" condition -- if the file begins with "AZII*", the first input record will be empty after the "chomp".)

You may need to play with that, e.g. if "AZII*" is supposed to be followed by a line-feed (or carriage-return + line-feed).

I don't understand why the output file names in the OP were set to "$num.txt" -- these are not text files. You say they are image files (tiff), so why not use the appropriate extension for the output files? Am I missing something in your description of the problem?