Sorry, I would find that useless.

I spend very little of my time displaying native Perl data types. When I do need to do it, I usually want to do it with Data::Dumper. When I don't want to do it that way, I generally have some target output format which is generally not going to fit your simple display above. Which I could write already as:

join "\n", map "$_=$ENV{$_}", sort keys %ENV;
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In fact even if your desired feature was added, I would deliberately never use it simply because looking up the key-word would be more energy than using the existing native flexibility to solve this problem simply and naturally.

Plus the fact that this join interface bears no relationship to the usual join strikes me as a design decision that can do nothing but lead to grief later.