Previous respondents have suggested efficient ways for various circumstances. As for the simplest way:

    use Quantum::Superpositions;
     
    if ($x == any(@list)) {
        ...
    }
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Note that the latest release of Q::S (just uploaded to the CPAN last week) is iterator-based and therefore much more efficient that previous versions. We have Steven Lembark to thank for that.

Note too that, whilst the performance of the grep- and hash-based solutions may improve slightly over time, the any solution will continue to improve significantly with future releases of Quantum::Superpositions, then with the new Perl6::Junctions module, and ultimately with the availability of any as a built-in feature of Perl 6.