my @subcategories = ("Physics");
my %unique_pages;

while (@subcategories) {
  my $next_subcategory = shift @subcategories;
  my @nodes = fetch_subcategory($next_subcategory);
  NODE: foreach my $node (@nodes) {
    if ($node->type() eq "subcategory") {
      push @subcategories, $node;
      next NODE;
    }
    $unique_pages{$node->unique_id()} = $node;
  }
}

foreach my $page (values %unique_pages) {
  my $page_content = fetch_page($page);
  # ... stuff here ...
}