sub getXCharacterPatterns{
  my $str=shift; #what does shift do here?
  my $pl=shift; #andd here
  my $slen=length($str);
  if ($pl>$slen){
     print "ERROR: $pl is greater than $slen";
  }

  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++) {
     $sstr=substr($str,$i,$pl); 
     $nu_hash{$sstr}++; 
  }
  return \%nu_hash;
}