Another way:

my %where;
map {$where{$_} = 1} keys %firsthash;
map {$where{$_} += 10} keys %secondhash;

foreach (keys %where) {
  print "$_: first list only\n" if $where{$_} == 1;
  print "$_: second list only\n" if $where{$_} == 10;
}
[download]

Edited d/t typing too fast and not rereading. Thx injunjoel.