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]

In reply to You're right... by chromatic
in thread String validation by Speedfreak

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