One way to do this, which uses JavaScript, but only once, is with a cookie. Read the resolution with JS, set a cookie with JS, after that, read it with Perl. You can probably do it with a Java Applet too.

But, like everyone else said, there's no foolproof way to do it and you'll only get yourself in trouble by trying. You need to re-examine the philosophy that says that knowing the res is the only fix for your problem.
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“Every bit of code is either naturally related to the problem at hand, or else it's an accidental side effect of the fact that you happened to solve the problem using a digital computer.” M-J D