It's been maybe 25 years since I wrote a prefix(algebraic) to postfix (RPN) routine to parse an algebraic expression. I would have to work on this for awhile to get it working.

Example using stack:  (2+3)*5 results in: push 2, push 3, add, push 5, multiply This requires an op precedence table.

I think you are better off with some form of Perl eval if the actual equation is unknown until runtime.

As another technique, for 8 variables, you can make an array of say 256 entries, use one bit for each variable, precalculate the results for the 8 variables and use a simple lookup table for the result.

This is also possible with a Perl hash table. Just calculate values where the 8 or whatever variables describe a "true" result.