Thanks for your attention. I've returned with details not available at home.

First I need to claim a bit of brain fade. The line "@complete = <INPUT>;" in the second snippet should have read "push @complete, <INPUT>;"--lostjimmy was exactly right with his comment.

Now back to the rat killing. When I run a pair of files (each about 89Mb) through these snippets, the first method runs about 105 seconds, the second only about four. If these files are concatenated externally to the script (i.e. into a single 180 Mb file) then the first runs in roughly 30 seconds while the second method still takes around four.

This is running on a 2.4 GHz Windows machine with 4Gb of RAM and the behavior still puzzles me.

As always, many thanks for your comments and thoughts.