Regardless of whether or not you're using CGI.pm (that's none of my business) you should leave the warnings on anyway. It's going to catch other problems eventually and that's a Good Thing.

Lots of gratuitous errors in the log will cause people to ignore the log...So...

If you honestly expect a lot of blank fields, and the prospect of checking for defined-and-then-a-value makes you nauseous, just pre-cook them all with a defined value (like the empty string) before you use them:

for(keys %USER) {
  $USER{$_}="" unless defined $USER{$_};
}
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