You can also profit the read of the main documentation for defined especially:

Use of defined on aggregates (hashes and arrays) is no longer supporte
+d. It used to report whether memory for that aggregate had ever been 
+allocated. You should instead use a simple test for size:

        if (@an_array) { print "has array elements\n" }
        if (%a_hash)   { print "has hash members\n"   }
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L*