Funny you should ask this now, as I just saw a very similar question on StackOverflow but for PHP, yesterday (posted 23 hours ago). Do you, by accident, know each other?

Anyway, what I'd think of is to store a simple expression, as text, in the database. You could do it in Perl and use eval. Or, my preference, you could write the expressions in a simpler language (which first of all doesn't have the '$' for the variables) than Perl, for example Javascript (and C and ... zillions of other languages), possibly even extended. For example your code could better, IMHO, be rewritten as:

  a eq "A" && p in (88, 89, 90, 91)
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Then all you have left to do is write a simple expression parser/evaluator, and I just happen to have written one, once. I've even converted it to Perl, here: Operator Precedence Parser. I see it doesn't parse functions (with parameters) (yet), which would be useful for implementing in(...), but that's not too hard to add. I might do that as an exercise, later today. <pEdit When I think about it, using/parsing "in" as a function is not right: it's actually an operator with a scalar on the LHS and a list on the RHS. A function with arguments would look similar, but would be used in a different position (as an expression instead of as an operator) so it ought to be parsed, and processed, differently.