A bit more clearly:

my $pages = $data->{query}{pages};
my ($page_id) = keys(%$pages);
my $page = $pages->{$page_id};
my $extract = $page->{extract};
...
[download]

Or if you don't need the page id:

my $pages = $data->{query}{pages};
my ($page) = values(%$pages);
my $extract = $page->{extract};
...
[download]

---

The layout suggests that multiple pages could be returned.

my $pages = $data->{query}{pages};
for my $page_id (keys(%$pages)) {
   my $page = $pages->{$page_id};
   my $extract = $page->{extract};
   ...
}
[download]

Or if you don't need the page id:

my $pages = $data->{query}{pages};
for my $page (values(%$pages)) {
   my $extract = $page->{extract};
   ...
}
[download]