Maybe it's time for the fabled use less

Good point. Maybe there just isn't a way for perl to detect how a particular variable could be optimized, but it would be possible if the user could decide.

things like your examples of encode and decode that take string and return it modifed in some way, ought to be written to use the aliasing pass-by-reference aliasing affects of @_ anyway.

Unfortunately I don't think it's realistic to demand that all modules be written this way. In the case of Encode, I'd rather use the module than my own memory-conserving code; and it's not convenient to change the module's source code. (I would probably even have to change it if "use less" worked, because it's lexically scoped afaik.)

(However I could encode the text line by line as Joost suggested.)