#!/usr/bin/env perl -l

use strict;
use warnings;

my $search = 'AAA';
my $offset = length $search;

while (<DATA>) {
    chomp;
    my $last_find_pos = length($_) - $offset;
    my $found_pos = rindex $_, $search, $last_find_pos;

    if ($found_pos == -1) {
        print "'$search' not found in '$_'";
    }
    elsif ($found_pos == $last_find_pos) {
        print "'$search' found at end of '$_'";
    }
    else {
        print "'$search' found before (but not at) end of '$_'";
    }
}

__DATA__
AAAAA
AAAAB
AAABB
AABBB
ABBBB
BBBBB

##</code><code>##

'AAA' found at end of 'AAAAA'
'AAA' found before (but not at) end of 'AAAAB'
'AAA' found before (but not at) end of 'AAABB'
'AAA' not found in 'AABBB'
'AAA' not found in 'ABBBB'
'AAA' not found in 'BBBBB'