In your code, each keyword can only point to one topic, which isn't very helpful.

if you stored a list of topics per keyword it would be much more flexible. As emacs can do everything there are going to be a lot of topics with the emacs keyword ;)

 for my $next_keyword ( @{ $next_topic->{keywords} } ) {

         # Store keyword in index.
         push @{$keyword{$next_keyword}} , $i;
 }
...
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