Although one of the examples in the POD suggests that trueColor() returns an image, it doesn't. The second example is correct. Ie. No return. So ignore the return code.

To verify it worked, call GD::IMage->trueColor(1);, then create your image, my $img = GD::Image->new( $x, $y ) or die ... and then call $img->isTrueColor() or die ....

Or, just pass a third argument to the constructor my $img = GD::Image->new( $x, $y, 1 );. The same works for all the other constructors also.