Can modify somehow be declared to only take a modifiable parameter, so that we can have a compile-time error (as happens with the core chomp)?

Playing Devil's Advocate, will this work?

chomp(foo(",",@list))
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Don't know? How about this?

chomp(join(",",@list))
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It seems, off the top of my naive head, that the argument to chomp would have to be an l-value to be guaranteed to be modifiable.

Is that what you want?