#! /usr/bin/perl -w

use strict;
use warnings;

my $a = "Input String: aaAabBAAa";

my $n = shift @ARGV;
$n --;

my $rx = qr/(.)(?=\1{$n})/i;
while ( $a =~ /$rx/g )
{
    print substr( $a, pos($a)-1, $n+1), "\n";
}