foreach my $item (keys %items) {
   my @set = sort @{$items{$item}};
   for my $first (0..$#set) {
      for my $second ($first+1..$#set) {
         $corr{ $set[$first] }{ $set[$second] }++;
      }
   }
}

for my $first (sort keys %corr) {
   for my $second (sort keys %{$corr{$first}}) {
      print "$first $second: $corr{$first}{$second}\n";
   }
}