Unnecessary Shell Calls

People can be very dogmatic about avoiding the shell
The program that makes these slides makes many shell calls
It converts many text files to HTML
- The conversion is slow
So it does the conversion only if the text file has changed:

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

     # later, convert each file in @SLIDES from text to HTML