This has been a debated discussion for awhile. When a person goes on international transfer (aka: country reassignment), will they get a new PERNR? The simple answer is yes! SAP has a internal hard check that does not allow the same pernr to be in multiple different countries.
With SAP version 4.7 extension II and above, SAP deliver a functionality called Management of Global Employees, a subset of Concurrent Employment functionality. This in itself introduces a whole new concept of tracking employees (see my help document of MGE on this website). With the concept of PERSON ID, it is a central ID now link to multiple PERNRs (also known as personnel assignments).
When a person goes on international transfer / country re-assignment, it is required by SAP for you to get a new personnel assignment number (PERNR). With that, in your configuration of personnel action, there is a check box flag to signify the action type is a country re-assignment.
In short, when a person goes on international transfer, you will be required by SAP to introduce to him/her a new PERNR. You can still track them as one individual through the PA side using PERSON ID or through the OM side using the central person concept.
Now it comes back to the population question. Can I "NOT" have SAP force me to create a new PERNR when a person goes on international transfer / country re-assignment. The answer is also yes. You can refer to SAP OSS Note 116007 for further detail. However, if you focus on the last phrase from SAP, this is NOT the recommended option and SAP will NOT support you if a problem does occur.