smtpd_recipient_restriction, you will not yet have the received that DATA. You will need to hang it off smtpd_data_restrictions

Neither of those is a "filter" in the general Postfix parlance. A filter is generally an external program or additional Postfix process (like a new smtpd) invoked either from main.cf as content_filter or through master.cf -- such as a program, Perl or shell script to which the entire email would handed off for processing. Postfix would expect it to be returned into the flow as processing or create a bounce. smtpd_data_restrictions reads after the SMTP conversation DATA command so it would not work for the OP's intention..

From what I can tell of the OP's intended task, (remember: "mail received in queue") he should be running his program as a "filter" in this sense, but even then - that may not be the best design decision - since all incoming mail would run through the Perl script , perhaps needlessly. Better would be to have mail to certain user accounts pipe through the program and get processed by a Perl script accordingly to find out whether they're already registered in the database. Maybe that's what he meant anyway..!