Now this is where it gets interesting... I've already added tracking to see where the ports get opened and closed, and I am definitely constructing and destructing (destroying?) correctly:

WARNING:
Opened port 1769 for FTP at c:/Perl/site/lib/Net/FTP.pm line 803.
WARNING:
Closed port 1769 for FTP at c:/Perl/site/lib/Net/FTP.pm line 929.

The code I'm using a little bit lengthy because I'm wrapping Net::FTP within another package to present a consistent interface to another system. Also, it's not the number of iterations that causes the problems as now I've added a counter onto the system to see how many iterations it gets to before crashing out, and it's breaking the 255 barrier easily, indicating that it is most likely closing sockets fine.

I've come across this before on Linux some time ago, with code that used to run for weeks and then suddenly fail to make a connection.