Thanks - that seems to be what is happening, but it differs from what some of the perl docs say. eg:
http://perldoc.perl.org/perlipc.html#Sockets%3a-Client%2fServer-Communication

"Listen The Listen parameter is set to the maximum number of pending connections we can accept until we turn away incoming clients. Think of it as a call-waiting queue for your telephone. "

or

http://docstore.mik.ua/orelly/perl/cookbook/ch17_03.htm

The numeric argument to listen is the number of un accept ed connections that the operating system should queue before clients start getting "connection refused" errors.