You can get rid of the hash refs, and use one big hash with exists for the flags. Then you can just assign undef for true, which is cheap. 
foreach my $item (LARGE_LIST) {
    $key = property($item);
    $record{$key}++;
    if(condition_is_true($item)) {
        $flag{$key} = undef;
    }
}

# later
if (exists $flag{$key}) {
    # flag is on
}

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In reply to Re: How to create a compact data structure? by perrin
in thread How to create a compact data structure? by bart

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