#!/usr/bin/perl

use strict;
use warnings;

my $formula = '((1 + 2) * 2) + (2 * 3)';

my %hash = ('*' => \&mal,
            '-' => \&minus,
        '+' => \&plus,
        '/' => \&div,);


while($formula =~ s/\(([^\(]*?)\)/calc($1)/eg){
}

print calc($formula);

sub calc{
  my ($part) = @_;
  my ($op1,$op,$op2) = $part =~ m!(\d+)\s*([\*\+\/\-])\s*(\d+)!;
  return 0 unless exists($hash{$op});
  return $hash{$op}->($op1,$op2);
}

sub mal{
    my ($op1,$op2) = @_;
    return $op1 * $op2;
}

sub minus{
    my ($op1,$op2) = @_;
    return $op1 - $op2;
}

sub plus{
    my ($op1,$op2) = @_;
    return $op1 + $op2;
}

sub div{
    my ($op1,$op2) = @_;
    return $op1 / $op2;
}

my ($part) = @_;