That's not a closure -- it's merely an anonymous sub reference.

This would be a closure being passed to a Tk function:

  sub make_counter { my $count = shift; sub { ++$count } }
  $mw->Button(-text => "count", -command => &make_counter(0)});
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That in and of itself shouldn't produce leaks -- the closure will be deconstructed along with the Button deconstruction. I would imagine you need be wary of reference cycles, however, like this (functionally meaningless):

  sub make_ref_reporter { my $thing = shift; sub { ref $thing } }
  $mw->Button(-text => "refthing", -command => &make_ref_reporter($mw)
+);
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Matt