Benchmarking chop/substr/split

[larryl]

I tried a few benchmarks in response to discussion around Big Willy's Frequency Analyzer. The code I tested and the results can be summarized by:

• chop: 909.92/s
  

  while ($_ ne '') {
     $letters{lc chop}++;
  }
  

• split_0: 519.41/s
  $letters{$_}++ for split//, lc;
• split_1: 481.23/s
  $letters{lc $_}++ for split//;
• substr: 475.51/s
  

  while ($_ ne '') {
     $letters{lc substr($_,-1)}++;
     substr($_, -1) = "";
  }
  

I have two questions:

1. I'm not that familiar with the for syntax used in the split_[01] tests. Can someone help me mentally parse those, and possibly explain why one would be faster than the other?
   
   
2. It seems odd that chop() benchmarks so much faster than substr($_,-1). I've seen the same in other stuff I've benchmarked. Wouldn't you think the compiler would treat substr($_,-1) as a special case?
(Ok, that was really just one question and one musing...)