Seen-Hashes as Lookup Tables

• Following Perl Cookbook, created 'seen-hashes' for each list

      # loop through master file to populate @master
      for (@master)  { $seen_master{$_}  = 1; }
  
      # read directory holding source files to populate @sources
      for (@sources) { $seen_sources{$_} = 1; }

• Then, ask whether master list is subset of source file list.

      $subset_status = 1;
      for (@master) {
          unless (exists $seen_sources{$_}) {
              $subset_status = 0;
              last;
          }
      }

• Wrote similar code to keep track of demonstration Perl scripts.

• Soon got tired of repeating code for seen-hashes and subsets.