The regex need to be called twice because unquoted words between : and , (eg Stan or 22) don't match

You can solve that using

$response =~s/([{:,])(\w+)(?=[:,}])/$1"$2"/g;
[download]

In 5.10+, you can even do

$response =~s/[{:,]\K(\w+)(?=[:,}])/"$1"/g;
[download]