Hashes are the best data structures to use when trying to create arrays with unique values, because the keys are guaranteed to be unique (duplicate keys are overwritten as opposed to being appended to the stack). When you start using hashes to do lookups for uniqueness, you've learned a valuable skill. Here's some commented code for how to do this:

# your arrays
my @a = qw(a c d);
my @b = qw(s d v);
my @c = qw(l d s);
my @d = qw(i c f);

# the hash we'll use to preserve uniqueness
my %unique = ();

# for every character in each of the defined arrays
for my $char (@a, @b, @c, @d) {
    # whenever we run into a character, we set its
    # corresponding value to 1 in the unique hash.  Even if we
    # run into the same character more than once, it'll only have
    # a single entry in our hash, as is the nature of hashes.
    $unique{$char} = 1;
}

# now, the keys in the %unique hash should be the unique
# values from those arrays.  We'll sort the keys for good
# measure
my @unique = sort keys %unique;

# @unique is now the unique elements of those arrays.  Kudos.
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Hashes are your friend (I use them a LOT when looking for duplicates in company data files). Learn to use them wisely and you'll appreciate perl's power and simplicity a lot sooner.

Or, for a quickie:

my %unique = map { $_=>1 } @a, @b, @c, @d;
my @unique = sort keys %unique;
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perl: code of the samurai