use strict;
use warnings;

my @list_a = ( 'a'..'g');
my @list_b = ( 'A'..'C', 'a'..'c');

my %count;
$count{$_}++ foreach (@list_a, @list_b);

foreach (sort keys %count) {
  print "$_ appears $count{$_} time.\n" if $count{$_} == 1;
}