Currently newly-added fields are stuffed at the end of the record, after 999. Bug 24480 is changing that to insert them in numerical order. However, as the methods on MARC::Record illustrate, there are two other ways that people want to insert fields: grouped by hundreds, so that instead of adding a new 502 after any existing 502s you add it after every existing 5xx field, or, before or after a specific other field.
This would be handy!