You should read the manual of Bio::SearchIO. It says:

Reads the next ResultI object from the stream and returns it. Certain driver modules may encounter entries in the stream that are either misformatted or that use syntax not yet understood by the driver. If such an incident is recoverable, e.g., by dismissing a feature of a feature table or some other non-mandatory part of an entry, the driver will issue a warning. In the case of a non-recoverable situation an exception will be thrown.

If this function returns undef, it is likely (although the manual is silent about it) to indicate an "End-Of-File" or "End-of-Stream" situation rather than an error as errors will throw an exception.

It is a common idiom to wrap methods such as object->next_SOMETHING in a while-loop to automatically run through all results and finish when there are no more "next" things to process.