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

print str_count('ABAABAAAA', 'AA'), "\n";

sub str_count {
    my ($str, $pat) = @_;
    my %substr;
    my ($p_len, $s_len) = (length $pat, length $str);

    my $template = ("A$p_len" . 'X' . ($p_len - 1)) x ($s_len - $p_len + 1);
    $substr{$_}++ for unpack $template, $str;
    return $substr{$pat};
}