Thanks for this code. (you're just missing a </code> terminating tag, but that's of little consequence in this short example).

I know almost zero about Gtk2 (and I can't find Gtk2::Combobox - do you mean Gtk2::Ex::ComboBox?) but I can say that $_[0] is likely to be undefined here. foreach just sets $_, not @_. Maybe try printing that, or using Data::Dumper or similar to dump the object, if it is one? Just a thought.

An SSCCE is always best.