Just because I like Quantum::Superpositions, I thought I'd provide an example.

use Quantum::Superpositions;

my @set = (1, 2, 3, 4, 5);

print "3 is a member!\n" if 3 == any(@set);
print "8 is a member!\n" if 8 == any(@set);
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Admittedly, this isn't the fastest solution (see the responses above for those), but it maps well to how I like to think about set operations. Sacrificing speed for clarity. :)