If you're not stuck to having to use POST (but instead use GET), then it could be as simple as:

read (STDIN, $query, $ENV{'CONTENT_LENGTH'});
$query .= '&cmd=_notify-validate';

# set up connection and do the request
my $socket = IO::Socket::INET->new( 'www.webserver.com:80' ) or die;
print $socket "GET /cgi-bin/webscr?$query HTTP/1.0\n\n";

# check return status
my $status = <$socket>;
die "Unacceptable status: $status" unless $status =~ /200/;

# lose the HTTP header
while (<$socket>) {
    last if /^\s+$/s;
}

# get the body
while (<$socket>) {
    $res .= $_;
}
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Liz