Ah. That one's easy. Consider the following snippet of Perl 5 code:

sub foo {
  my $bar = shift;
  my %baz = @_;
  ...
}
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If you get rid of @_ and the defaulting behaviour of shift/pop et al how do you do a programmatic conversion to Perl 6?

Of course, once you've got it autoconverted to perl 6 you'd go through and change it to

sub foo($bar, %baz) {
  ...
}
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But having running code which you can test is important.