The fragment you have shown will not compile. The following will compile:

... and will return the "last" key of hash %$hash_ref. But that would seem somewhat meaningless, as the keys of a hash do not have any specific order. Any apparent order of the keys may change when the elements of the hash are changed, or a different Perl is used.

Did you perhaps mean (...)[-1]?

That's a list slice. The expression in the parens is evaluated in list context. The expression in the square brackets must return a list of indexes. The slice returns the elements of the returned list identified by the indexes. Negative indexes are relative to the end of the list, where -1 is the last element of the list.

( grep { $_ % 4 == 0 } 1..10 ) )[-1]   # The last num divisible by 4.
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Here's another guess along the lines of "Maybe if I add some more stuff, it'll be a valid statement":

>perl -wMstrict -le
"my @hash_ref = ({ qw(a 1  b 2) }, { qw(c 3  d 4) });
 ;;
 print keys   %{ $hash_ref [-1] };
 print values %{ $hash_ref [-2] };
"
cd
12
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