I believe your question relates to writting code that can get past a proxy rather than trying to use PPM through a proxy.
That being the case, take a look at the CPAN module, Net::HTTPTunnel.
I believe it will get you started and has references for more reading as a bonus
Comment on Re: Access the Internet though a Proxy with Perl
Another much appreciated response. It took me a little longer to reply to this than the others because before I could follow it up my employer actually wanted me to do some work.
This was a very interesting thing to look up and has opened my eyes if not solved my problem. I have played with the example as listed in the documentation on CPAN. I think I am successfully authenticating with the proxy because if I do something like...
...it does work if I put a valid user ID and password in and doesn't if I make something up. However I can't work out how to use this to make the jump to getting through the proxy to read a webpage.
So much to learn, so little brainpower and time to do it with.