If you had written @a{(undef) } = () you'd have gotten the same result.
But (undef) and () are the different things. The first one is not empty list, but the second one is empty.
BTW the following code gives the identical results with all versions of the perl (no warninigs, empty hash) 
perl -wle 'use Data::Dumper; my %a; @a{()} = (); print Dumper(\%a)'

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In reply to Re^2: using "my" in hash slice by ccn
in thread using "my" in hash slice by ccn

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