#!/usr/bin/perl

use strict;
use warnings;

my $formula = '((1 + 2) * (2 -1)) + (2 * 3) + 5';

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


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

print calc($formula);

sub calc{
  my ($part) = @_;
  while($part =~ s!(\d+)\s*([\*\/])\s*(\d+)!subcalc($1,$2,$3)!eg){};
  while($part =~ s!(\d+)\s*([\+\-])\s*(\d+)!subcalc($1,$2,$3)!eg){};
  return $part;
}

sub subcalc{
  my ($op1,$op,$op2) = @_;
  return 0 unless exists($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;
}