Thanks for pointing out Win32::GuiTest which complements Win32::GUI quite nicely though GetCaretPos() comes up kinda empty.

Anyway, the problem turned out to be my use of the wrong event handler. onMouseDown made GetSel() return erratic values. Asking GetSel() in the context of onMouseUp always returns the correct value for the insertion point.

Win32::GUI docs: yeah, best to read the XS and MSDN...