A variation:

use strict;
use warnings;

my @critters = qw(
                  ant camel snake camel llama
                  cow sheep herring camel llama
                  );
my %rec_of;
my %dup_rec_of;

for my $critter (@critters) {
    $dup_rec_of{ $critter }++ if $rec_of{ $critter }++;
}
printf "%s\n", join ', ', sort keys %dup_rec_of;
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which prints:

camel, llama
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Cheers