If you use KDE and can live with a really (really) basic interface, you could try using kdialog (I assume there is something similar for Gnome).

$val = `kdialog --title "Question" --inputbox "What is $a x $b?"`;

if ($val == $correct) {
  system("kdialog --msgbox 'Correct!'");
}
else {
  system("kdialog --msgbox 'Wrong!'");
}
[download]

I've used it for a math quiz thingy which was fairly successful. :-)