my %your_hash = (
   aa => 1,
   bb => 2,
   cc => 3,
   dd => 4,
   ee => 2,
   ff => 5,
   gg => 3,
   hh => 2,
);

use Var::Pairs;

my %keys_for_each_value = invert(%your_hash);

use Data::Dumper 'Dumper';
warn Dumper \%keys_for_each_value;