Unable to parse Variables with PayPal IPN script

#!/usr/bin/perl
 
# read post from PayPal system and add 'cmd'
read (STDIN, $query, $ENV{'CONTENT_LENGTH'});
$query .= '&cmd=_notify-validate';
 
# post back to PayPal system to validate
use LWP::UserAgent;
$ua = new LWP::UserAgent;
$req = new HTTP::Request 'POST','https://www.sandbox.paypal.com/cgi-bin/webscr';
$req->content_type('application/x-www-form-urlencoded');
$req->content($query);
$res = $ua->request($req);
 
# split posted variables into pairs
@pairs = split(/&/, $query);
$count = 0;
foreach $pair (@pairs) {
($name, $value) = split(/=/, $pair);
$value =~ tr/+/ /;
$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
$variable{$name} = $value;
$count++;
}
 
# assign posted variables to local variables
$name_first = $variable('first_name');
$name_last = $variable('last_name');
$address_street = $variable('address_street');
$address_city = $variable('address_city');
$payment_status = $variable{'payment_status'};
$payment_amount = $variable{'mc_gross'};
$payment_currency = $variable{'mc_currency'};
$txn_id = $variable{'txn_id'};
$receiver_email = $variable{'receiver_email'};
$payer_email = $variable{'payer_email'};
 
#assign variables for email 
$status = $res->content;
 
if ($res->is_error) {
# HTTP error
}
elsif ($res->content eq 'VERIFIED') {
# check the $payment_status=Completed
# check that $txn_id has not been previously processed
# check that $receiver_email is your Primary PayPal email
# check that $payment_amount/$payment_currency are correct
# process payment
 
open (MAIL, "|/usr/sbin/sendmail -t -oeq") || die("Could not send
email.\n");
    print MAIL "To: payment\@mydomain.co.nz \n";
    print MAIL "From: paypal\@mydomain.co.nz \n";
    print MAIL "Subject: Payment Verification\n\n";
    print MAIL "$name_first \n";
    print MAIL "$name_last \n";
    print MAIL "$address_street \n";
    print MAIL "$address_city \n";
    print MAIL "$payment_status \n";
    print MAIL "$payment_amount \n";
    print MAIL "$payment_currency \n";
    print MAIL "$txn_id \n";
    print MAIL "$receiver_email \n";
    print MAIL "$payer_email \n";
 
close (MAIL);
 
}
elsif ($res->content eq 'INVALID') {
# log for manual investigation
 
open (MAIL, "|/usr/sbin/sendmail -t -oeq") || die("Could not send
email.\n");
    print MAIL "To: payment\@mydomain.co.nz \n";
    print MAIL "From: paypal\@mydomain.co.nz \n";
    print MAIL "Subject: INVALID IPN Notification\n\n";
    print MAIL "IPN returned: $status\n\n";
    print MAIL "Here is the raw IPN string:\n";
    print MAIL "$query\n";
 
close (MAIL);
 
}
else {
# error
}
print "content-type: text/plain\n\n";