I think this should be a good exercise and learning experience for you. There are many solutions ...

Solution 1

#!/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 ....
[download]

And the output is -

:A:
AA for apple
A for apple
:B:
BB for ball
B for ball
:C:
C for ....
[download]

Solution 2
A more perl-ish solution, with regular expressions

#!/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
...
[download]

Solution 3
Another variant, read the file into a scalar and do search and replace in one go:

#!/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
...
[download]

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