keys(%hash) returns false if the hash is empty. This is what you want.
[Upd: Actually just
%hash in scalar context is better if you don't need a count.
]
You'll never want to used defined on hashes and arrays. It reports on an unimportant internal property of those variables (whether or not some memory is allocated for data). This usage is even deprecated. Why aren't you using warnings?
$ perl -wle'my %a; print defined(%a)?1:0'
defined(%hash) is deprecated at -e line 1.
(Maybe you should just omit the defined()?)
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