I'm not entirely sure of the correctness of my answer, but my limited experience suggests that once your script is running, it no longer needs the file and you can edit it all you like. The next time the script is run it will pick up the new version, providing that there are no errors. I recommend using perl -c scriptname to catch compilation errors.

I guess it could depend on how your background jobs are called. system and backticks should work the way I expect. Is it something else?