Interesting. This seems to be another scalar vs. array vs. arrayref change between Tk800 and Tk804. Try the following code in Tk800 and Tk804. The first one shows the stringified array reference, while the second one shows the array.

perl -MTk -e '$txt = tkinit->Text->pack; $txt->insert("end", [1,2,3]);
+MainLoop'
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So to be portable: always write my($result) = $w->method instead of my $result = $w->method (e.g. for Tk::Listbox::curselection) and do not use the arrayref forms where it is possible (e.g. in Tk::Canvas::createLine).