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

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

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

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