use DBI;
  my $dbh = DBI->connect('dbi:File(RaiseError=1):');
  $dbh->do("CREATE FUNCTION $_ EXTERNAL") for qw(Prof Class);
  sub Prof  {[ [qw(pid pname)],[qw(1 Sue )],[qw(2 Bob)],[qw(3 Tom )] ]}
  sub Class {[ [qw(pid cname)],[qw(1 Chem)],[qw(2 Bio)],[qw(2 Math)] ]}
  $sth = $dbh->prepare("SELECT * FROM Prof NATURAL JOIN Class");
  $sth->execute;
  $sth->dump_results;