Perhaps some variation on `xwininfo ...`? On this 'ere machine, xwininfo -root returns the correct info. How well it works with Xinerama I don't know. Don't rely on it being portable, as the format of the data it returns may differ.

If you really want to be portable, look at X11::Protocol:

use X11::Protocol;
my $x = X11::Protocol->new();
print $x->{screens}->[0]->{width_in_pixels} .
      ' x ' .
      $x->{screens}->[0]->{height_in_pixels} .
      "\n";
[download]

I haven't been able to test this with Xinerama, but some variation on that theme should do the trick.