# cat re1
use warnings;
use strict;

undef $/;
my $tout = <DATA>;
my $z;

while ($tout =~ /((?:DES|TN).+?(DATE|ZONE)[^\n]+\n\s*\n)/sg) {
         print "\n\n", ++$z, ":\n", $1;
}

__DATA__
DES  MAIL
TN   001 0 02 00
TYPE SL1
CDEN DD
CUST 0
KLS  1
FDN
TGAR 0
LDN  NO
NCOS 0
     09
DATE  9 MAR 2000

TN   001 0 02 01
     05 RLS
     06 TRN
     07 AO3
     08
     09
ZONE 002

TN   001 0 02 01
     05 RLS
ZONE 001
     07 AO3
     08
     09
DATE  9 MAR 2000

##</code><code>##

# perl -w ./re1


1:
DES  MAIL
TN   001 0 02 00
TYPE SL1
CDEN DD
CUST 0
KLS  1
FDN
TGAR 0
LDN  NO
NCOS 0
     09
DATE  9 MAR 2000



2:
TN   001 0 02 01
     05 RLS
     06 TRN
     07 AO3
     08
     09
ZONE 002



3:
TN   001 0 02 01
     05 RLS
ZONE 001
     07 AO3
     08
     09
DATE  9 MAR 2000