#!/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 ....