ive researched about shift, but only found broad answers that were no help, i cant avoid not understanding shift because it pops up everywhere!!

also, my friend pat showed me a sliding window for a long file, to find Xcharacter patterns,( it would look somethin like this) ACGTTGCAAGCTGTAAA, and there are a couple places are confusing me, help me? :) code is below, thanks in advance 

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;
}

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i know my question is quite lengthy, but bear with me :-),

In reply to trouble understanding code, and shift() w.o args by DanielM0412

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