foreach my $date (keys %counts){
  my $overallcount = $overallcounts{$date};
  foreach my $word (keys %{$counts{$date}}){
    $counts{$date}{$word."_average"} = sprintf("%.2f", 100 * $counts{$date}{$word} / $overallcount);
  }
  $counts{$date}{"_overallcount"} = $overallcount;
}

print Dumper \%counts;