Also worth considering: if your lookups are (or lend themselves to be adapted to) a simple boolean truth; vec can reduce the space requirement to a fraction of that used by either arrays or hashes and yields lookup time that also beat arrays: 

$vec = ''; vec( $vec, $_*20, 1) =1 for 1 .. 1e5;;
print total_size \$vec;;
250056

$found = 0; say time; vec( $vec, $_, 1 ) and ++$found for 1 .. 20e5; s
+ay time; say $found;;
1359141761.02608
1359141761.35157
100000

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In reply to Re^2: Array vs. Hash for sparsely integer-indexed data (bit vectors) by BrowserUk
in thread Array vs. Hash for sparsely integer-indexed data by puterboy

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