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            53214                                         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            48180                  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 05/15/2012   AJT9#######
    31#######       44xxx xxx AVE                     .00 05/16/2012 06/15/2012   AJT92######                      NOV
    0             C  LINCOLN PARK,MI xxxxx        2045.00 05/05/2012                        1                      3
    0             S  LINCOLN PARK MI                  .00            48146                  1                      12