If your window has 400x300 dimensions then

$mw->geometry('400x300+'.int(($mw->screenwidth-400)/2).'+'.int(($mw->s
+creenheight-300)/2));
[download]

will help you.

But I see in some systems geometry works strange.
1. If I use KDE4.5.2 without 3d then all works fine.
2. If I use Compiz on Kubuntu 10.10 then some time windows position is random. In old versions of Kubuntu all was fine. May be this is temporary.
3. Some problems was on Windows with StrawberyPerl.