This seems to be a special two-parameter form for the binding (it uses an anonymous array), where the first parameter (or array value) is the sub as before, and the second furnishes the parameters for the sub. Tkx::Ev just does that. See http://www.tkdocs.com/tutorial/concepts.html for this explanation:

Tkx lets us provide command callbacks as just a Perl function (the first four), or as a two element array (the last case). The first element is the Perl code to be called, while the second array element specifies parameters to pass to that code. The function "Tkx::Ev()" will expand its parameter ("%x %y" in this case) when the callback is invoked, which will perform the percent substitutions. These then are passed as parameters to our function.