My above code has a nasty habit of allowing things it shouldn't depending on the order of the conditions. Assuming that 'disallow' rules trump 'allow' rules, here is a quick fix.

sub get_menu_items {
    my ($rules,@conditions) = @_;
    my %results;
    
    for my $cond ( @conditions ) {
        next unless exists $rules->{$cond};
        for $item ( @{$rules->{$cond}->{allows}} ) {
            $results{$item}++;
        }
    }
    
    for my $cond ( @conditions ) {
        next unless exists $rules->{$cond}; 
        for $item ( @{$rules->{$cond}->{disallows}} ) {
            delete $results{$item};
        }
    }
    return keys %results;
}
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