my %hash;
  my $value = "foo";
  my @cats = qw(a b c d);

  my $p = \%hash;
  foreach my $item (@cats) {
      $p->{$item} ||= {};
      $p = $p->{$item};
  }
  $p->{_val} = $value;

  use Data::Dumper;
  print Dumper(\%hash);