At least from my vague reading of the documentation in its original Klingon, it seems that ->clipboard_get() is supposed to be called on an existing object and not on a class method.

I haven't done enough Gtk3 programming to know what the customary object would be on which one would call the method though.

Update: Grepping the distribution for "clipboard", I find t/zz-GtkTextBuffer.t, which might or might not have relevant usage:

SKIP: {
  skip 'clipboard stuff; missing annotations', 0
    unless Gtk3::CHECK_VERSION (3, 2, 0);

  my $clipboard = Gtk3::Clipboard::get(Gtk3::Gdk::Atom::intern('clipbo
+ard', Glib::FALSE));

  $buffer -> paste_clipboard($clipboard, $buffer -> get_end_iter(), TR
+UE);
  $buffer -> paste_clipboard($clipboard, undef, TRUE);
  $buffer -> copy_clipboard($clipboard);
  $buffer -> cut_clipboard($clipboard, TRUE);

  $buffer -> add_selection_clipboard($clipboard);
  $buffer -> remove_selection_clipboard($clipboard);
}
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