#!/usr/bin/perl -w
use strict;

my $letter ='a';

while (<DATA>) 
{  
    print_record();
    print "\n";          # record spacer #
    $letter++;
}

# adds an alpha character to the id that's like <1>
# except weirdly enough for a.
#
sub print_record
{
    s/<(\d+)>/<$1$letter>/ unless $letter eq 'a';
    print;
    while (defined($_ = <DATA>) && $_ !~ /^\s*$/)
    {
        s/<(\d+)>/<$1$letter>/ unless $letter eq 'a';
        print;
    }
}

=prints:

<1>66543
<2>2009
<3>TY
<4>-
<6>
<7>TOM
<8>2 - DUNDAS
<8>100 - MAIN
<9>ONT 00001
<10>
<11>000-000-0000
<12>

<1b>12345
<2b>1999
<3b>PP
<4b>XLX - TEST
<6b>Comm
<7b>
<8b>1 BLVD
<9b>MA 00001
<10b>CITI, INC.
<11b>000-000-0000
<12b>

<1c>8887
<2c>2000
<3c>
<4c>TEST
<6c>
<7c>JOE
<7c>MARY
<8c>100 MAIN
<9c>CT 02221
<10c>BH, INC.
<11c>871-000-0000
<12c>YEAR

<1d>test
<2d>2087
<3d>XX
<4d>-
<6d>
<7d>DOM
<8d>2 - CHT
<8d>1000 - CAR
<9d>ONTA X0001
<10d>
<11d>000-000-0000
<12d>
<13d>
<14d>CB
=cut

__DATA__
<1>66543
<2>2009
<3>TY
<4>-
<6>
<7>TOM
<8>2 - DUNDAS
<8>100 - MAIN
<9>ONT 00001
<10>
<11>000-000-0000
<12>

<1>12345
<2>1999
<3>PP
<4>XLX - TEST
<6>Comm
<7>
<8>1 BLVD
<9>MA 00001
<10>CITI, INC.
<11>000-000-0000
<12>

<1>8887
<2>2000
<3>
<4>TEST
<6>
<7>JOE
<7>MARY
<8>100 MAIN
<9>CT 02221
<10>BH, INC.
<11>871-000-0000
<12>YEAR

<1>test
<2>2087
<3>XX
<4>-
<6>
<7>DOM
<8>2 - CHT
<8>1000 - CAR
<9>ONTA X0001
<10>
<11>000-000-0000
<12>
<13>
<14>CB