Also not to be excluded entirely is ... uhh ... an array of integers that is searched sequentially each time.

That is a surprise winner for my 'Worst Advice of the Month (January, 2013)' prize.

... set up a few short test-runs ...

I'd suggest you take your own advice before doling it out to others.

NB: The following uses 1/10 the number of values and 1/10 the number of lookups than the tests above for arrays, hashes and bit vectors, because it takes so long to complete.:

@a = map $_*20, 1 .. 1e4;;
say total_size \@a;;
320176

$found=0; say time; for my $tgt ( 1 .. 20e4 ) { $tgt == $_ and ++$foun
+d and last for @a; }; say time; say $found;;
1359149323.02034
1359149541.80419
10000
[download]

Using that as the basis for estimation, the total size will be 3.2MB -- 12 times that required by the bitvector solution.

And the lookups that took hashes 1 second; arrays 2/5ths of a second and bitvectors 1/3rd of a second; would take: 25 days!

It is really hard to see any circumstance when this would be a viable solution.