How do I password protect a single perl script?

I have an editing script which I want to protect. It has several elements all of which are contained in one sub-routine. Is there a way to allow a single user to access the editor subroutine without having to re-write any of it's code?

I've tried having a password script which calls the sub-routine. It worked and brought up the editor, but when I tried to use it everything went back to the log-in. I'm not a Perl expert so have probably missed something incredibly simple - usually the case