#!/usr/bin/perl -w
use strict;
my $chunk = 2**20;  # try 1MB to start but it may be faster to go bigger/smaller
my $infile = 'c:/test.txt';
my $outfile = 'c:/out.txt';
open IN, $infile or die "Can't open $infile $!\n";
open OUT, ">$outfile" or die "Can't open $outfile $!\n";

my $buffer;
my $partial_line = '';
my $start = time;

while (read(IN, $buffer, $chunk)) {
    # we should only process full lines so we trim off the partial line 
    # that we inevitably get at the end of our read and save it into $2
    $buffer =~ s/^(.*\n)([^\n]+)\z/$1/s;
    # add last partial line to front of buffer
    $buffer = $partial_line.$buffer;
    # save the current partial line for next loop so we can add it back on
    $partial_line = $2 || '';
    # make changes
    $buffer =~ s/this/that/g;
    print OUT $buffer;
}

print "Took ", time - $start, " seconds\n";

close IN;
close OUT;