#!/usr/bin/perl
use warnings;
use strict;

my $haystack = shift;
my %needles;

undef @needles{@ARGV};

my @len = sort {$a<=>$b} keys %{{ map { (length,0) } keys %needles }};

my $pos = 0;
my $hlen = length $haystack;
while ($pos + $len[0] <= $hlen) {
    for my $L (@len) {
        last if $pos + $L > $hlen;
        my $substr = substr($haystack, $pos, $L);
        $needles{$substr}++ if exists $needles{$substr};
    }
    $pos++;
}

use Data::Dumper;
print Dumper \%needles;

##</code><code>##

$ ./973643.pl 'AAAAA AAACACA CAACAAA' AAA AAC ACA CAA
$VAR1 = {
          'AAC' => 2,
          'ACA' => 3,
          'CAA' => 2,
          'AAA' => 5
        };