How can I tell whether an array contains a certain element?

There are several ways to approach this. If you are going to make this query many times and the values are arbitrary strings, the fastest way is probably to invert the original array and keep an associative array lying about whose keys are the first array's values. 

    @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
    undef %is_blue;
    for (@blues) { $is_blue{$_} = 1 }

Now you can check whether $is_blue{$some_color}. It might have been a good idea to keep the blues all in a hash in the first place.

If the values are all small integers, you could use a simple indexed array. This kind of an array will take up less space: 

    @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
    undef @is_tiny_prime;
    for (@primes) { $is_tiny_prime[$_] = 1; }

Now you check whether $is_tiny_prime[$some_number].

If the values in question are integers instead of strings, you can save quite a lot of space by using bit strings instead: 

    @articles = ( 1..10, 150..2000, 2017 );
    undef $read;
    grep (vec($read,$_,1) = 1, @articles);

Now check whether vec is true for some $n.

Please do not use 

    $is_there = grep $_ eq $whatever, @array;

or worse yet 

    $is_there = grep /$whatever/, @array;

These are slow (checks every element even if the first matches), inefficient (same reason), and potentially buggy (what if there are regexp characters in $whatever?).