As a "i just now made this up rule of thumb", use braces when parens won't do.

I think I'd rather the rot was "try braces when parens didn't do what you thought they would".

Ditto for curlies and parens when dealing with hashes. Although looking at why, maybe with Data::Dumper, as you say, or looking at what perl told us when we tried to run the thing!!!, is always best. Otherwise we might as well be monkeys bashing away at our keyboards.