while (my $w1 = <FILE> and my $w2 = <FILE2>) {
  if ($w1 lt $w2) {
    print "\n";
    $w1 = <FILE>;
  }
  elsif ($w1 gt $wt) {
    print "\n";
    $w2 = <FILE2>;
  }
  else {
    print $w1;
  }
}
# Leftover lines in either file?
print "\n" while <FILE>;
print "\n" while <FILE2>;