You don't understand how $report_data{$domain}{$provider} is a reference?

If not, if you were to define things all at once it would be like this:

my %report_data = (
  $domain => { 
    $provider => {
      'error' => '0',
      ... # other stuff here
    }
  }
}
[download]

The reason it's a reference is because you have a scalar value (hash value $report_data{$domain}) pointing to a hash reference.

From perldoc -f perldata:

...
All data in Perl is a scalar, an array of scalars, or a hash of scalars
...
Although a scalar may not directly hold multiple values, it may contain a reference to an array or hash which in turn contains multiple values.
...

A hash isn't a scalar, it's a hash, but a reference is a scalar, so it can hold that in the value of the hash where the key is $provider.