I agree with everything that Corion and marto have written. Checkmarx is giving you the error, so Checkmarx also needs to tell you how to fix it.

As I indicated, above I was only guessing what the problem might be, and in this case I can also only guess that maybe Checkmarx wants you to confirm that $err_rate really does contain a float. But Perl is notoriously hard to parse, so I have no idea what code the tool would accept for that check, so again, you'd have to look at the Checkmarx documentation or ask them. And if Corion is right that %% is the problem, then the tool is giving you a false positive (which proves my point), and you need to talk to Checkmarx support.