Actually, the docs say:

    undef $ref;
    if (exists $ref->{"Some key"})    { }
    print $ref;         # prints HASH(0x80d3d5c)
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"This surprising autovivification in what does not at first--or even second--glance appear to be an lvalue context may be fixed in a future release."

Translating 'autovivication' from the ancient Latin I get: "of bringing itself to life," so I take it to mean "self-define" in the context.

As for a workaround, I would expect using 'defined()' instead of 'exists()' ought to do it.