#!/usr/bin/perl -l

use strict;
use warnings;

my @old=split ' ', <DATA>;
print "@old";

while (<DATA>) {
    my @cur=split;
    print "@cur[0,1]" eq "@old[0,1]" ?
      $cur[2] : "@cur";
    @old=@cur;
}

__END__
foo bar baz
foo fred  baz
foo  fred burz
bar baz burp