sub STORE {
  my ($tied_hash, $value, @keys) = @_;
  my ($x, $y, $z) = @keys; # to stay consistent with above example
  if( $x == 'foo' ) { ... }
  ...
}

sub FETCH {
  my ($tied_hash, @keys) = @_;
  my ($x, $y, $z) = @keys; # to stay consistent with above example
  if( $x == 'foo' ) { ... }
  ...
}