TMTOWTDI

  use strict;
  print "Enter your monitor's resolution?\n";
  chomp ($_ = <>);
  $_ = /^\s*(\d+)\s*x\s*(\d+)\s*/i or die;
  print "The resolution is $1 x $2\n";
[download]

If you're using a browser, why not just dump some Javascript in there, like this:

  <script language="JavaScript">
    var width = window.screen.availWidth
    var height = window.screen.availHeight
    alert(width + "*" + height)
  </script>
[download]

Consult the Netscape and Microsoft object model documentation on the following properties:

   screen.width
   screen.height
   screen.availWidth
   screen.availHeight
   document.body.clientWidth
   document.body.clientHeight
[download]