Is it possible to have multiple commands preceeding an unless test?

Use do:

do {print "Program Error!"; exit(0)} unless [case is true];
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Although, personally, I would find the prefix variant of unless clearer if you're doing more than one thing.

While I'm at it, I have a short script that replaces a specific line in a file. Is there a better way to do this than what I'll pseudo code below?

Take a look at Tie::File - dead useful :-)