Hashes (such as %$data) can only contains scalars for values. So $data->{followersperdate} can't possibly hold a hash as you believe. Note the curlies in the Data::Dumper output. It holds a reference to a hash.

my $followers = $data->{followersperdate};
for my $date ( keys %$followers ) {
    my $id = $followers->{$date};
    ...
}
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