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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In reply to Re: what is keys %$hash_ref [-1] by ikegami
in thread what is keys %$hash_ref [-1] by Anonymous Monk

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