You two indeed pointed me into the right direction! It's now working:

The method need an additional argument. As first, I pass in which "base" object I am operating on "$this->{tab2}->{maingrid}->", then I tell Wx through the next (needed!) argument, which sub-part of this object it should hide. So I changed my strategy and now I am operating on the 3-column Sizer called maingrid, of which I hide the third column:

$this->{tab2}->{maingrid}->Hide(2);

(the hint at $bool and at that "Hide and Show affect objects managed by the sizer." was invaluable!)