Even if the contents of the hash are the same each time you call the sub, it's still a different hash. To take the most trivial example:

sub get_hashref {
  my %hash;
  return \%hash;
}
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it creates a new my %hash each time it's called, since the old one went out of scope and no longer exists¹. If you want to prove this to yourself, create a trivial sub like that and call it twice, storing both hashrefs. Add some data to one hashref, then look in the other - it will still be empty.

(There are ways to have it return references to the same hash every time it's called, such as

my %hash;
sub get_hashref2 {
  return \%hash;
}
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but defectdata() apparently doesn't do this.)

¹ Not entirely true, in that the previous hash still exists and can be accessed by the previously-returned hashref, but it has become anonymous and is no longer my %hash. Read up on "closures" for more about how this works (and how it can be used to do some pretty powerful stuff).