Sorry for the delay - 2004 was before my time on PerlMonks...

Today, xrandr would be the tool of choice:

qwurx [shmem] ~ > xrandr
Screen 0: minimum 320 x 200, current 2944 x 1200, maximum 4096 x 4096
VGA-0 connected 1920x1200+0+0 (normal left inverted right x axis y axi
+s) 518mm x 324mm
   1920x1200      60.0*+
   1280x1024      75.0  
   1152x864       75.0  
   1024x768       75.1     70.1     60.0  
   832x624        74.6  
   800x600        72.2     75.0     60.3     56.2  
   640x480        72.8     75.0     66.7     60.0  
   720x400        70.1  
   0x0             0.0  
DVI-0 disconnected (normal left inverted right x axis y axis)
LVDS connected 1024x768+1920+0 (normal left inverted right x axis y ax
+is) 0mm x 0mm
   1024x768       60.0*+   60.0  
   800x600        60.3     56.2  
   640x480        59.9
[download]

I'd parse that as follows, to get the current Xinerama screen portion a toplevel is in. Crude hack...

my ($W, $H); # width and height of current display

{
    (my $screennumber = $ENV{DISPLAY}) =~ s/:(\w+\.)?//;
    my $screenfound = 0;
    open my $fh, '-|', 'xdpyinfo' or die "Can't run xdpyinfo: $!\n";
    while(<$fh>) {
        $screenfound++ if /screen #$screennumber:/;
        next unless $screenfound;
        if(/dimensions:\s+(\d+)x(\d+)/) {
            ($W,$H) = ($1,$2);
            1 while <I>;
        } 
    }
    close $fh;
}

# get_area - get the current Xinerama screen portion
#
# input: $window - a Toplevel Tk window
# return:
#   $ox: offset x
#   $oy: offset y
#   $w:  width
#   $w:  height

sub get_area {
    my $window = shift;
    chomp (my $xrandr = `which xrandr`);
    my ($ox, $oy, $w, $h);
    if ($xrandr) {
        open my $fh, '-|', $xrandr or die "Can't run xrandr: $!\n";
        my @a;
        while(<$fh>) {
            /\bconnected (\d+)x(\d+)\+(\d+)\+(\d+)/
                and push @a, { w => $1, h => $2, x => $3, y => $4 };
        }
        if (1 < @a) {
            # find current offset
            my @g = $window->geometry =~ /(\d+)x(\d+)([\+-])(\d+)\+(\d
++)/;
            $g[3] += $W-$g[0] if $g[2] eq '-';
            splice @g,2,1;
            my %g; @g{qw(w h x y)} = @g;
            my ($min,$max)
              = ($a[0]->{w} >= $a[1]->{w} && $a[0]->{h} >= $a[1]->{h})
              ? reverse @a : @a;
            if(
              $g{x} >= $min->{x} && $g{y} >= $min->{y} &&
              $g{x}+$g{w} <= $min->{x}+$min->{w} &&
              $g{y}+$g{h} <= $min->{y}+$min->{h} ) {
                $ox = $min->{x};
                $oy = $min->{y};
                $w  = $min->{w};
                $h  = $min->{h};
            } else {
                $ox = $max->{x};
                $oy = $max->{y};
                $w  = $max->{w};
                $h  = $max->{h};
            }
        }
    }
    ($ox, $oy, $w, $h);
}
[download]

Heh... does this post make sense, more than five years later? ;-)

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