Having spent some time writing a largish program in perl 6 (and then porting it to perl 5 to make sure that the design was sound), I found that one of the persistent mistakes I kept making in my perl 5 code was trying to use . all the time instead of ->.

However, the hardest thing to give up was the given/when syntax, which is just lovely. Being able to use the -> $foo, $bar {...} syntax to make anonymous functions of known arity was rather cool too, if a little more obscure...