#!/usr/bin/perl
use CGI; 
$q = new CGI; 
print $q->header, $q->start_html;
#-------------------------------------------------------------------------------
$message = $q->param('message'); #retrieve state 
$balance = $q->param('balance'); 

if ($message) { # state is not empty 
  $function = $q->param('function'); #read user input 
  $amount = $q->param('amount'); 

  if ($function =~ /deposit/i) { # update the state (with the user input) 
    $balance += $amount; 
  } elsif ($function =~ /withdraw/i) {
    $balance -= $amount; 
  } else {
    
  } 
  $message = "Your balance is: " . $balance; 

} else { # could not find a message so state is empty, initialize it 
  $message = "Welcome, your balance is: 0 (zero)"; 
  $balance = 0; 
} 
#---------------------------------------------------------------------------
print qq{
  <form> 
    $message <p> 

    Choose action: <select name="function"> 
      <option value="nothing"> Click me!
      <option value="deposit"> Deposit
      <option value="withdraw"> Withdraw
    </select> <p> 

    Amount: <input type="text" name="amount"> <p> 

    Press <input type="submit" value="Proceed"> when ready.

    <input type="hidden" name="balance" value="$balance"> 
    <input type="hidden" name="message" value="$message"> 

 </form> 
}, $q->end_html;