Like stevo said, it is also mentioned in perlreftut (Make rule 2)

If you write just "[]", you get a new, empty anonymous array.
If you write just "{}", you get a new, empty anonymous hash.

my %hash =
(
      'debt-1' => {
                            'debt_id' => 10,
                            'mobile' => 11,
                            'address1' => 12
                        },
);

my $hash_ref =
{
      'debt-1' => {
                            'debt_id' => 10,
                            'mobile' => 11,
                            'address1' => 12
                        },
};
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