Thank you all for your suggestions.

As oiskuu demonstrated, the bit mapped index -- as suggested by Corion, Salva, Choroba & hdb -- is hands down winner in the performance stakes.

By using vec and string-wise boolean operations (Salva,Choroba) rather than numeral ops, I don't have to worry about the size of the small integers outgrowing the platform integer size, which is a slight but possible consideration.

Mixing code from various solutions, this is what I'm using:

use constant MAX   => 20;
use constant BLANK => chr(0) x ((MAX / 8) + 1);
...
    my $key = BLANK;
    vec( $key, $_, 1 ) = 1 for @$_;
    $cache{ $key } = $_;
...
    my @subsel = grep{ ( $_ & $mask ) eq BLANK } keys %cache;

[download]

And that's it. Thank you all.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re: Efficient selection mechanism? (Thank you all) by BrowserUk
in thread Efficient selection mechanism? by BrowserUk

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