In file1, is word1..word4 unique (only 1 line has any specific word1..word4)? Is it feasible to read in the whole file? If so, my approach would be something like:

  my %file1;
  while (<FILE1>) {
    # Capture first four words, and everything after the next 3 words 
+(numbers)
    my ($k1, $k2) = /(\w+(?:\s+\w+){3})(?:\s+\w+){3}(.*)/;
    # Index by k1, store k2 and line
    $file1{$k1} = [$k2, $_];
  }
  while (<FILE2>) {
    my ($k1, $k2) = .... # same regex as before
    if ($file1{$k1}) {
       # print file1 line and file2 line
       print $file1{$k1}->[1];
       print;
       # Check if k2's are equal
       if ($file1{$k1}->[0] ne $k2) {
           print "K2's are not equal! (see lines printed above)\n";
       }
    }
  }
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I've probably misunderstood some of what you want, but I think the parsing of the keys could be a significant help to you.

Update: fixed regex as per nobull's note.