Mail Folder Analyzer Revisited

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The profiler says that main::letter_histogram is consuming most of the CPU time

     %Time ExclSec CumulS #Calls sec/call Csec/c  Name
      63.7   0.830  0.829    109   0.0076 0.0076  main::letter_histogram
      13.8   0.180  0.180      1   0.1800 0.1800  Mail::Util::read_mbox

A 20% speedup in this one function would reduce the program's run time by 1/8

     sub letter_histogram {
       my $strdex = (length $_[0])-1;
       $letter_hist{substr($_[0],$_,1)}++     for (0..$strdex);
     }

Perhaps loop over the characters directly
- Instead of looping over 0 .. $strdex and indexing the string?

     sub letter_histogram {
       $letter_hist{$_}++     for split //, $_[0];
     }

     Before                    After

     real    0m2.739s          real    0m5.379s
     user    0m1.270s          user    0m2.410s
     sys     0m0.040s          sys     0m0.040s

Well, that didn't work