Thanks to all who responded. I should have mentioned earlier that my file is 5meg in size with 169312 lines - larger than I want to slurp into an array all at once.

It's been a bit of a trick to go through and figure out what the different replies have in common. The most obvious trick (to others, at least!) was $foo = <FH>; which apparently pops a single line off of FH, sticks it in $foo, and updates FH so the next read will return the next line in the file.

I feel pretty silly; I'm used to code like this

while ( $line = <FH> ) { 
  # do stuff to $line
}
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but I didn't realize I could $foo = <FH>; outside of a while-ing context. While this is a worthwhile thing to know, for me the larger lesson was using a different approach.

tadman said it best: you could just re-work how you're using Perl. Indeed. I was thinking I needed to read three lines, process them, then skip backwards two lines and repeat the cycle. It seems that everyone else realized I just needed to prime my array by reading two lines, then push a third line onto the array, process the lot, shift the first one off, and start pushing and processing again.

For me the neat part about SOPW and the various answers is finding out which answer (if any) the poster decided to go with. In my case, I'm going with the answer that looks the most like my current code - an adaptation of tadman's response. This isn't based off of benchmarks or how easily I can incorporate the changes into my existing code. Perhaps it just seems closest to my Perl idiolect (awkward pidgin that it is).

#!/usr/bin/perl

use strict;
use warnings;

my @three;

# Stoke it
chomp ($three[0] = <DATA>);
chomp ($three[1] = <DATA>);

while(<DATA>) {
  chomp ($three[2] = $_);

  # Do stuff to process @three
  print join ("\t", @three), "\n";

  # Rotate
  shift(@three);
}

__DATA__
1
2
3
4
5
6
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Thanks again- blyman
setenv EXINIT 'set noai ts=2'

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