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

my %letter;
while (<DATA>) {
  print("$_"), next if /^\s*$/; # print and skip empty lines
  my $c = substr $_, 0, 1;      # get first character
  if (! exists $letter{$c}) {   # have we seen it before?
    $letter{$c}++;
    print ":$c:\n";
  }
  print;
}

__DATA__
AA for apple
A for apple
BB for ball
B for ball
C for ....

##</code><code>##

:A:
AA for apple
A for apple
:B:
BB for ball
B for ball
:C:
C for ....

##</code><code>##

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

my %letter;
foreach (<DATA>) {
    s/^(.)/$letter{$1}++ ? $1 : ":$1:\n$1"/e;
    print;
}

__DATA__
AA for apple
A for apple
...

##</code><code>##

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

local $/;
my $data = <DATA>;

my %letter;
$data =~ s/^(.)/$letter{$1}++ ? $1 : ":$1:\n$1"/emg;
print "$data";

__DATA__
AA for apple
A for apple
...