Interesting question, and I really want to hear a comprehensive answer.

As of my personal opinion, here is my vision on where to start:

• wget utility from cygwin package has option --limit-rate=RATE which limits download bandwidth. Perform some RTFS if you're familiar with C
• as to current load of modem/eternet-card is a question where it is hard to give a correct answer, because it is hard to distinguish network load from one to another. I mean, say, your 100MBit ethernet card loaded by 5% because of small traffic, but then you connect to 256Kbit Internet, which is overloaded by outside download.
  What I would recommand anyway - get information from netstat command with different parameters, and your script to decide what to do with certain statistical results...
• may be just use wget --limit-rate=[rate-specify-here] command from your script?

I would be hapy to see perl answer to the first item.

Courage, the Cowardly Dog.