This does the trick using hashes

tachyon

my @test = ("camel","aplaca","peguin","aplaca","monkey","camel","camel
+");
my (%duplicate,%test,@redundant);

for (@test) {
    $duplicate{$_} = 1 if defined $test{$_};
    $test{$_} = 1;
}

@redundant = keys %duplicate;
print "@redundant\n";
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