Let's make it a little more readable, then:

if ($string =~ m!^(\w{4})$!) {
    $string =~ tr/[a-z]/[A-Z]/;
    # or $string = uc $string;
    
    # do something
    } else  {
    # do some error
}
[download]

Since the OP wants four alphanumerics (and they are merely *likely* to arrive in lowercase), we'll use \w.

The reason I suggested using tr/// instead of uc is that, in my testing, uc() failed on a scalar starting with a digit. Now it seems to work correctly. Let's go back to:

if ($string =~ s!^(\w{4})$!uc($1)!e) {
    
    # do something
    } else {
    
    # do some error
    # print "Location: $error_url"
}
[download]