From the end of the recsep section of Tie::File you quoted

Inserting records that contain the record separator string is not supported by this module. It will probably produce a reasonable result, but what this result will be may change in a future version. Use 'splice' to insert records or to replace one record with several.

I tried using Tie::File to add "\n" at the end of a line on Windows 7 and it did not concatenate two lines as the OP claims. In notepad every record was on a separate line. In notepad++ there was an extra blank line inserted. Since "\n" is not the record separator for Win7 it seems resonable the module would put it in like other text.

The point of Tie::File is the convenience of treating a file like an array so the OP seems to miss this by fiddling with record separators.