use strict;
use warnings;
use Data::Dumper;

my %in_hash = (
  jibber => [
              {id =>  1, score => 3, name => 'foo'},
              {id =>  5, score => 1, name => 'bar'},
              {id => 22, score => 6, name => 'baz'},
            ],
  jabber => [
              {id =>  3, score => 1, name => 'boo'},
              {id =>  5, score => 3, name => 'bar'},
              {id => 12, score => 2, name => 'zib'},
              {id => 22, score => 2, name => 'baz'},
            ],
);

my %tmp;
for my $key (keys %in_hash) {
   for my $href (@{$in_hash{$key}}) {
      my $id = $href->{id};
      if (exists($tmp{$id})) {
         $tmp{$id}->{score} += $href->{score};
      }
      else {
         $tmp{$id} = $href;    
      }
      $tmp{$id}->{keys}->{$key}++;	  
   }
}

my %out_hash;
for my $id (keys %tmp) {
   my $keys = $tmp{$id}->{keys};
   delete $tmp{$id}->{keys};
   my $new_key = join(' ', reverse sort keys %$keys);
   push(@{$out_hash{$new_key}}, $tmp{$id});
}

print Dumper \%out_hash;

##</code><code>##

C:\tmp>erick.pl
$VAR1 = {
          'jibber' => [
                        {
                          'name' => 'foo',
                          'score' => 3,
                          'id' => 1
                        }
                      ],
          'jabber' => [
                        {
                          'name' => 'boo',
                          'score' => 1,
                          'id' => 3
                        },
                        {
                          'name' => 'zib',
                          'score' => 2,
                          'id' => 12
                        }
                      ],
          'jibber jabber' => [
                               {
                                 'name' => 'baz',
                                 'score' => 8,
                                 'id' => 22
                               },
                               {
                                 'name' => 'bar',
                                 'score' => 4,
                                 'id' => 5
                               }
                             ]
        };