I don't know if the Perl bindings can reach it, but the Win32 API for determining this is (GetKeyState(VK_LBUTTON) < 0) (or GetAsyncKeyState()). Mouse buttons are just seen as more Shift-like keyboard elements in this regard.

The difference between the two APIs is esoteric and you need to know a little how message loops are implemented to fully grok it; typically, you want to just use the first one.