disclaimer: Without looking into the Tk source m'self...

What the line $viewer->bind("<3>", [\&post_popup_menu]) does is call the method 'bind' in (whatever class "ref $viewer" tells you) with the parameter list: ($viewer, "<3>", $arrayref). That $arrayref is the new array reference created by the  [  ] operator, which in turn contains only the CODE ref (created by the  \& operator(s?) ) to post_popup_menu.

Note that a CODE ref cannot have arguments associated with it; the arguments are provided by the (eventual) caller. (If you wanted to pass certain args, you could use a closure to wrap the callback, e.g.  [ sub { &some_function ($whatever, @_); } ])

When your callback function (post_popup_menu) is called, it looks like it receives a parameter telling it 'who called,' which from what you said here, would be $viewer. (the 'shift' pulls the next - or in this case, first - argument to the subroutine off @_.) This object then appears to have stored the X and Y location of the event for which you were called - in this example, "<3>", a right-click (mouse button 3 click; could be on the left ;-) ).

I'd assume, then, that $viewer_menu is the menu, and to "Post" it means to "stick it" at the X,Y coords given.

Those Data::Dumper outputs should be interesting; you might want to poke around the XEvent structure to see what else you can do this way.