The way to assign no elements to a hash is 
%h = ();

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After this statement defined %h returns false. Whereas 
%h = undef;

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is assigning a single key to the hash (whose value happens to be undef, and thus coerced into the empty string '') with no corresponding value, hence the warning. But after this assignment the hash is no longer empty, so defined %h returns true.

In reply to Re: hash assignment wierdness by Errto
in thread hash assignment wierdness by Bro. Doug

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