I don't know if this is the "best" way, but one route would be to use Tie::File. It basically "represents a regular text file as a Perl array". And since it does not store the entire file into memory, you can use this for large files without major concern about memory usage - although the 8KB size you mentioned in another post isn't too big to store in memory.
If you use Tie::File, grabbing the last 20 lines is simply grabbing the last 20 elements of the array.
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