my %nu_hash=(); #why is this an empty hash? (is he storing patterns 
+in a hash?)
[download]

It is an empty hash because my creates a new empty variable.    Some people feel that assigning an empty list to an already empty variable is a good practice.

  my $sstr =""; #why is this an empty string?
[download]

Actually, this variable is declared in the wrong scope.    It should be:

  for ( my $i = 0; $slen >= $i + $pl; $i++ ) {
     my $sstr = substr( $str, $i, $pl ); 
     $nu_hash{ $sstr }++; 
  }
[download]

Which would also be better written as:

  for my $i ( 0 .. $slen - $pl ) {
     my $sstr = substr( $str, $i, $pl ); 
     $nu_hash{ $sstr }++; 
  }
[download]