use Data::Dumper;

use Data::Difference qw(data_diff);

my @letters = qw( a a a b b b c c c );

my @word = qw( a a a b b);

my @diff = data_diff( \@letters, \@word );

print Dumper( \@diff );

for (@diff) { print $_->{a} }

__END__

$VAR1 = [
          {
            'a' => 'b',
            'path' => [
                        5
                      ]
          },
          {
            'a' => 'c',
            'path' => [
                        6
                      ]
          },
          {
            'a' => 'c',
            'path' => [
                        7
                      ]
          },
          {
            'a' => 'c',
            'path' => [
                        8
                      ]
          }
        ];
bccc
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Update: "If you don't know the algorithm quickly, it's better to check out a module." (Karl Goethebier, PerlMustard)

Regards, Karl