Perl Style: Use Hashes for Sets

Consider finding the union and intersection of two unique arrays @a and @b:

    foreach $e (@a) { $union{$e} = 1 }
    foreach $e (@b) {
        if ( $union{$e} ) { $isect{$e} = 1 }
        $union{$e} = 1;
    }
    @union = keys %union;
    @isect = keys %isect;

This would be more idiomatically written as:

    foreach $e (@a, @b) { $union{$e}++ && $isect{$e}++ }
    @union = keys %union;
    @isect = keys %isect;

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