$n=4;
$str="abababbcad";
%count;
while($str =~ m[(?=([abcd]{$n})).]g){
                   ++$count{ $1};     
               }

##</code><code>##

$count{$1}=1

##</code><code>##

$str1="abad";
$str2="aaab";
$n=2;