$ perl6 -e 'my @a = 1, 2, 3; push @a, 4; say ~@a'
1 2 3 4
$ perl6 -e 'my @a := 1, 2, 3; push @a, 4; say ~@a'
Method '!fill' not found for invocant of class 'Int'
  in 'List::push' at line 2617:CORE.setting
  in main program body at line 1