You bounce an email by sending a message back to the server that sent it. There are a number of possible messages but user unknown at this domain is what you want. See Mail::Message::Construct::Bounce for a very simple interface to bounce messages in perl. Note this link fails to work properly. Go to search.cpan.org and search for 'MAIL::Bounce' and it will bring it up OK

However what you really want to do is not have a catch all email address. Typically this will be (for sendmail) set up in the virtusertable like this:

[root@www root]# cat /etc/mail/virtusertable
@domain.com             username
...
[download]

cd /etc/mail/
vi virtusertable

Now replace the @domain.com with

address@domain.com    username
corporate@domain.com  username
bitbucket@domain.com  username
etc@domain.com        username
spam@domain.com       spam

Finally run make to recompile the .db file
[download]

Now any mail to domain.com that does not match one of the listed addresses will bounce. Note multiple 'fake' addresses go to the one username mail box.

It is easier to let the MTA bounce messages for you. Otherwise you will have to parse you mail box using the Mail::Box and friends. Note Mail::Box is the biggest pain in the ass to install as it has *lots* of dependencies, and I do mean lots. Each of these has more. Suggest use CPAN to put it on for you.