$DOC1="1.txt";
$DOC2="2.txt";

open (DOC1,$DOC1);
@words1=<DOC1>;
close (DOC1);

open (DOC2,$DOC2);
@lines2=<DOC2>;
close (DOC2);

foreach $line2(@lines2) {
 @words2=split / /, $line2;
  foreach $word2(@words2) {
   s/$word2/$words1[0]/;
   splice (@words1, 0, 1);
  }
 print @words2;
}