locate the source directory and get the list of files there
 establish the name of the destination directory, and create it if necessary
 if $source_dir has subdirectories { 
   create them as needed in the $dest_dir
 }
 for $infile ( @sourc_files ) {
   $outfile = $infile;
   $outfile =~ s{$source}{$dest};  # might be unnecessary?
   open(IN,"<","$source_dir/$infile");
   open(OUT, ">","$dest_dir/$outfile");
   while (<IN>) {
      s/$source/$dest/g;
      print OUT;
   }
   close IN;
   close OUT;
 }