use Tree::RB::XS;

for(@files){
  open IN, $_;
  while(<IN>){
    /^( [^\t\n]+ )\t( [0-9]+ )$/x; # avoid backtracking
    $f{$1}+=$2;
  }
}
my $t= Tree::RB::XS->new(compare_fn => 'int', allow_duplicates => 1);
while (my ($word, $count)= each %f) {
  $t->insert($count, $word);
}
my $iter= $t->iter;
printf OUT "%s\t%s\n", reverse $iter->next_kv while !$iter->done;