Not sure where JSON is involved, what you show is not JSON.

To just remove the quotes in this case, you can do:

$string = "90'+7'";
$string =~ s/'(\+\d+)'/$1/a;
[download]

(assuming it is always + not - and an integer). If it could happen more than once in your string, change /a to /ga.

If you mean you want the addition actually performed, leaving you with 97, do:

$string = "90'+7'";
$string =~ s/(\d+)'(\+\d+)'/$1+$2/ae;
[download]

(assuming the first number is a not too large unsigned integer).