> + }
> + $changed{$params{year}}{$params{month}} = 1;
>
-> $changed{$params{year}} is a scalar but `()` is a list. I think you want `{}`
+> `$changed{$params{year}}` is a scalar (you can tell because it starts with the
+> `$` sigil) but `()` is a list. I think you want `{}`
> (a scalar that is a reference to an empty anonymous hash).
>
> However, that whole `if` block can be omitted, and you can just use
> `$changed{$params{year}}{$params{month}} = 1;`, because Perl will automatically
-> create $changed{$params{year}} as a reference to a hash (the term to look
+> create `$changed{$params{year}}` as a reference to an empty hash, in order to
+> put the pair `$params{month} => 1` in it (the term to look
> up if you're curious is "autovivification").
>
> --[[smcv]]