subclassed function modifying original object

[JupiterHost.Net]

Hey group,
A trifle stuck trying to modify the original object in a subclassed 
function like so:

For instance (simple example):
my $gd_obj = new GD::Image(100,100);
my $new_gd = $gd_obj->foo();
sub GD::Image::foo {
   my $objgd = shift;
   return $objgd->copyRotate90();
}
Works like a charm, now what I'd like to do is make it so that if you 
call it in void context:

$gd_obj->foo();
instead of
my $new_gd = $gd_obj->foo();
that it modifies $gd_obj instead of returning the copy:
sub GD::Image::foo {
   my $objgd = shift;
   my $new_img = $objgd->copyRotate90();
   return $new_image if defined wantarray;
   # do what to modify original $gd_obj since !defined wantarray ???
}
I've tried modifying $objgd and ${$objgd} but to no avail, any ideas?
TIA
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