Many thanks for your comments and suggestions. I have taken them all on board.

I was going to ask about passing the statement handle around - it didn't feel right. Neatly solved.

I also agree that a for loop is now easier to read than the map. Again, many thanks.

sub get_category_loop {
  
  my @cat_loop_hrefs = @{
    $dbh->selectall_arrayref(
      sql_category(), 
      {Slice => {}}
    )
  };
  
  for my $cat_href (@cat_loop_hrefs){
    $cat_href->{subject_loop} = 
      get_subject_loop(delete $cat_href->{category_id});
  }
  
  return [@cat_loop_hrefs];

}
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