Okay first, s/refferral/referral/g;  : )

Then, add use strict; and use warnings; at the top of the file. If this is a CGI, you might also add use CGI::Carp('fatalsToBrowser') for more helpful error messages.

What I think what you're seeing is runaway recursion on build_reffers. Can you do away with the join statement and the subsequent return @ref? After all, you're just calling the subroutine in void context, so that code is just needlessly complicating things.

Update: Anomo++ for the explanation of why it recurses forever.