#!/usr/bin/perl
# silent11
# 11/16/2004

use strict;
use warnings;

my $paypal_email = "";
my $paypal_pass  = "";
my $notify_from  = "";
my $notify_to    = "";
my $balance_file = "balance.txt";
my $balance      = return_balance();

# # # Get notified via email (or cell phone txt msg)
# # # of Paypal balance changes

notify_me( $balance ) if compare( $balance );

# # # Subs # # # 

sub return_balance {
  use WWW::Mechanize;
  my $browser = WWW::Mechanize->new();
  $browser->get("https://www.paypal.com/");
  $browser->form(1);
  $browser->field("login_email", $paypal_email);
  $browser->field("login_password", $paypal_pass);
  $browser->click();
  $browser->follow("click here to reload");
  my $html = $browser->content;
  # Please don't laugh at the regex ;^) It works!
  $html =~ /(.*history">\$)(\d+\.\d+) (USD)/s;
  return $2;
}

sub compare {
  use Tie::File;
    # First time to use Tie::File
    # very cool!
    tie my @balance, 'Tie::File', $balance_file || die $!;
    my $new_balance = shift;
    my $cur_balance = $balance[0];
    $balance[0] = $new_balance;
  untie @balance;
    return 1 if ($new_balance != $cur_balance);
}

sub notify_me {
  use MIME::Lite;
  my $msg = MIME::Lite->new(
            From     => $notify_from,
            To       => $notify_to,
            Subject  => 'Paypal Balance',
            Data     =>  $balance
            );
  $msg->send || die $!;
}
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Update: changed http to https per zentra's suggestion.