Math::Expr does exactly what you want. Release date is 2001-10-01, so it's fairly up to date.

     NAME
         Math::Expr - Parses mathematical expressions


     SYNOPSIS
         use Math::Expr;

         SetOppDB(new Math::Expr::OpperationDB('<DBFileName>'));
         $e=Parse("a+4*b-d/log(s)+f(d,e)");

     DESCRIPTION
         Parses mathematical expressions into a tree structure. The expressions
         may contain integers, real numbers, alphanumeric variable names,
         alphanumeric function names and most other characters might be used
         as operators...

From the pod. That's no reason not to write your own as an exercise :)

After Compline,
Zaxo