There are two ways to specify recipients with sendmail. Either pass them on the command line, or use the '-t' option and it will pull them out of the To:, Cc:, Bcc: headers. Since you want to have the recipients in the To: header anyway and it is safer to put them in the header, use '-t' option.
Now, the To: header separates the list of recipients with commas. One way is to concat onto the recipient string.
$recipient .= ', somebody@somewhere' if $FORM{'ckusno'};
Another way is to have a list of recipients and join them together.
push @recipients, 'somebody@somewhere' if $FORM{'ckusno'};
my $recipients = join(', ', @recipients);
However, you should not put this script up on the web. It will be hacked. It will be used to send spam through your server. It will get you and your employer is lots of trouble. It may allow the server to be compromised and taken over. That might not be a bad thing because the hackers might delete this script.