There are a lot of people on #perl with a lot of opinions. While I agree with people who would use a join approach, of those two I would pick the first because it has the smallest number of "moving parts". It is therefore a conceptually simpler line of code.

I'm not a big one for labels. Just because one approach is labelled "functional" doesn't mean that it is better. Use whatever is simplest for the job at hand. Besides, the person doing the labelling doesn't know what functional means - the presence of the .= operator makes neither version functional.

The only advantage of the second one is that you can chain together maps to add complexity. However even that advantage is questionable since chains of maps lead to so much complexity that the maintainer is likely to balk while trying to figure out what it is doing (let alone what it is supposed to do).