Quantum::Superpositions might give the most readable way to do this:

use Quantum::Superpositions;

if ( $x == any(1,3,7) ) {
    ...
}
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A hash is better than an array or quickly checking of whether an item is included. If you can keep the set of candidates around as a hash with candidates as keys instead of an array of candidates, you can check whether a key is one of the candidates with exists.