The following pattern matches a function foo() which may contain
balanced parentheses as the argument.

$re = qr{ (                    # paren group 1 (full function)
            foo
            (                  # paren group 2 (parens)
              \(
                (              # paren group 3 (contents of parens)
                (?:
                 (?> [^()]+ )  # Non-parens without backtracking
                |
                 (?2)          # Recurse to start of paren group 2
                )*
                )
              \)
            )
          )
        }x;