It's a key, with the undefined value as its corresponding value. If you have warnings enabled you'll get a warning "Odd number of elements in hash assignment"
If you use that hash in list context you'll get a list of a single element which contains that string HASH... By the way, note that it is a string, it's not an actual reference. The reason is that in Perl, hash keys are always treated as strings; other scalars (numbers, references, undef) are stringified when you use them as a hash key.
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