Unnecessary Shell Calls

        if (! -e ".bak/$filename" || system("cmp -s $filename .bak/$filename")) { 
          print STDERR "*";
          push @SLIDES, $filename;
        }

I had a big argument with some programmers about this
- They said "Calling cmp all the time is wasteful"
- "You should maintain a file with MD5 checksums for each file"
My opinion:
- Writing a lot of MD5 garbage when cmp works just fine is even more wasteful
- To do 180 cmps takes under 4 seconds
- The MD5 thing might be faster
- But it can't be more than 4 seconds faster
  - Sufficient unto the day is the evil thereof