{ ... } creates a hash initializes it with the contents of the braces, and returns a reference to the hash. It makes no sense to assign that to a hash variable.

When you use parens instead of curlies, it builds a list. When a list is assigned to a hash variable, it's treated as a sequence of key-value pairs. In the code you presented, there's only one value in the list (a reference to a hash), so it's treated as a key after issuing a warning.

That explains the problem you were already told about. You have a second problem, though.
grep($thing, @{...})
means
grep { $thing } @{...}
Since $thing has a true value, that's the same as just
@{...}
You probably want to use
grep($_ eq $thing, ...)