And of course, the hash approach:

my %in;
undef @in{1, 2, 3..9, -12};

print "$i is a 1, 2, 3, 4, 5, 6, 7, 8, 9, or -12\n"
   if exists $in{$i};
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This method's advantage is that you can reuse the hash if you need to check if multiple variables are in the set, while still being extremely efficient.