my $str = "BEBEBEHUHUHUJJFAFALL";
my $length = 2;
my $index = 0;
my %cnt;        # hash to store the counters
while (1) {     # infinite loop
    my $substring = substr $str, $index, $length;    # getting a substring of $length length, starting at offet $index (initially 0)
    last if length($substring) < $length;            # exiting the infinite loop if we are at the end of the string
    $cnt{$substring}++;                              # increasing the counter for the substring
    $index += $length;                               # increasing the offset by $length
}