open(SENDMAIL, "|/usr/lib/sendmail -t");
print SENDMAIL "To: webmaster\nFrom: $mailfrom\n\nThis is the body\n");
close(SENDMAIL)

##</code><code>##

$mailfrom="joe@spammer.com\nTo: spam-recipient1@aol.com\n\nThis is the spammer's message body\n.\n";
</pre>
to cause you to send arbitrary mail to arbitrary users.