It looks like you are doing a lot of ref to hash and hash to ref transformations. This might be simpler:

modifyHash(\%my_hash);

sub modifyHash
  { my $hash_ref = shift;

    # Do something to the hash 
    $hash_ref->{'test'}=1;

    # no need for return, the original hash has been modified
    # the return value could be a success/failure flag
  }
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