With all the talk of map and side effects, I thought it might be nice to show you a version using map with no side effects. I actually kind of prefer this:

my @fields = qw/this that the_other/;
@fields = map { qq("$_") } @fields;
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Sure, it copies @fields instead of modifying it in place, but I doubt that makes a significant different in performance (most of the time). I think this is much clearer in expressing your intentions than using s/// with the for loop.