The former builds a frelling huge temporary list on the stack and then copies that into @array in one swell foop, while the later pushes one line at a time.

Of course a better question might be: why are you loading an entire 1.6G file into RAM? It's more efficient to process things a line at a time (or maybe a record at a time, depending on the structure) if at all possible rather than slurping it all in at once. There may also be things you can do such as writing to a Berkeley DB file or an RDBMS and then process using that instead.

But that would take more information about exactly what you're trying to do with your 1.6G.