sub BackAndForth {     
     my ($filename) = @_;


      #  make multi-dimensional hash here ... it looks like
      #  $ALLDATA{$filename}{$key1}{$key2}{$key3} = [@array]
      
      return (%ALLDATA);     
}

      my %ExternalHash = &BackAndForth('filename.csv');
         %ExternalHash = &BackAndForth('filename2.csv');