teamassociated:

Part of the difficulty appears to be that the file may have some lines you want to discard. Since the $. variable keeps track of the lines read and not the lines wanted, using the modulo operator on it is going to fail. However, the modulo operator idea is a *good* one, you just need to keep track of the lines you want to keep yourself:

my $cnt_keepers=0;
while (my $ln = <$f>) {
    next if ....

    # Now we've got a line we want to keep
    ++$cnt_keepers;
    if ($cnt_keepers % 5 == 0) {
        ... every fifth line processing ...
    }
    ...
}
[download]

Another way you could do it is to simply accumulate the lines you want, and when you have five, process the lot of 'em:

my @records;
while (my $ln = <$f>) {
    next if ...

    push @records, $ln;

    if (@records==5) {
        print "A $records[0]\n";
        print "B $records[1]\n";
        print "C $records[2]\n";
        print "D $records[3]\n";
        print "E $records[4]\n";
        print "F $records[5]\n";
        @records = ();
    }
}
[download]

It may be coincidence, but it seems like the first record of each group always has a 'Z' (?record type indicator?) around column 20. If it's not a coincidence, then another way to handle it would be to accumulate records until you find a line with a 'Z' marker, then process all the records you have, and then put the new record into the accumulator.

Update: Fixed a code tag.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

Perhaps the following, which uses List::MoreUtils's natatime to grab five elements at a time from a list of file lines, will be helpful:

use strict;
use warnings;
use List::MoreUtils qw/natatime/;

my $it = natatime 5, <DATA>;

while ( chomp( my @lines = $it->() ) ) {
    my $letter = 'A';
    my $acctNum = do { $lines[0] =~ /\s+(\d+)\s+(\d+)/; $1 . $2 };
    push @lines, " acctnum=$acctNum";

    print for map { s/\s+/$letter++ . ' '/e; "$_\n" } @lines;
}

__DATA__
Place your data here...
[download]

Update: Added a chomp; removed the non-destructive substitution modifier (/r) and a print "\n"; line.

Looks awesome, but I get errors: it does not like the /r, but does fine with /e except the output is all 11111.

# perl -c foobar
Bareword found where operator expected at foobar line 12, near "s/\s+/
+$letter++ . ' '/er"
syntax error at foobar line 12, near "s/\s+/$letter++ . ' '/er "
foobar had compilation errors.
root@facs04ap [/var/adm/scripts]
# perl -v

This is perl, v5.8.8 built for aix-thread-multi
[download]

code from u

use strict;
use warnings;
use List::MoreUtils qw/natatime/;

my $it = natatime 5, <DATA>;

while ( my @lines = $it->() ) {
        my $letter = 'A';
        my $acctNum = do { $lines[0] =~ /\s+(\d+)\s+(\d+)/; $1 . $2 };
        push @lines, " acctnum=$acctNum";

        print for map { s/\s+/$letter++ . ' '/er } @lines;
        print "\n";
}

__DATA__
    50######    8 Z  Axxx ,xxxxxxx           19793.43 02/02/2012 06/08
+/2012      32989.05
    037######    S  JOYCE xxxxxxS                  .00 06/06/xxxx     
+                                          0
    0             7  1471 xxxx xxxx x #101              .00 02/08/2012
+ 06/15/2012                                    NOV
    0             U  MILWAUKEE, WI xxxxx         32989.05 02/08/2012  
+                                             3
    3#####        MILWAUKEE WI                     .00            5321
+4                                         12
    50#####    8 Z  Axxx ,Nxxxxx                  75.00 05/27/2012 06/
+05/2012       1845.00
    03######3    E  Nxxxx Axxxx                     .00 09/xx/xxxx 06/
+05/2012   MYT#####
    7#######    S  156xxx xxxxx BLVD                .00 06/06/2012 06/
+14/2012   MYT########                      NOV
    0             C  TAYLOR,MI 48180              1845.00 05/27/2012  
+                      3                      3
    37######     S  TAYLOR MI                        .00            48
+180                  3                      12
    50#####    9 Z  ALxxxx Rxxxx ,SUSxxx        75.00 05/05/2012 05/15
+/2012       2045.00
    0000000000    E  SUSE Axxxxx Rxxxx             .00 03/xx/xxxx 05/1
+5/2012   AJT9#######
    31#######       44xxx xxx AVE                     .00 05/16/2012 0
+6/15/2012   AJT92######                      NOV
    0             C  LINCOLN PARK,MI xxxxx        2045.00 05/05/2012  
+                      1                      3
    0             S  LINCOLN PARK MI                  .00            4
+8146                  1                      12
[download]

Your version of Perl doesn't support the non-destructive substitution modifier (/r). Use the following:

print for map { s/\s+/$letter++ . ' '/e; $_ } @lines;
[download]

Will make that change in the original posting.