I can't see anything obvious you've done to cause a cycle or long-lived closure (though someone more familiar with the Gtk bindings might do).

Personally, I don't like the closures over your two globals, but that's unlikely to be the source of your issues.

As an experiment, try to move some of the font logic (and anything else that wouldn't need to be computed each time) out of draw_text.

It may be that the Gtk2 (or Gtk2::Pango) module does some resource caching that you're not aware of. If so, try using Devel::Leak to find those leaks.

-David