While I lean towards the flat list, I dislike more having to remember which convention is required in a particular piece of code. So I tend to support both forms:

sub new {
  my $class = shift;
  my $args = (ref $_[0] eq 'HASH') ? shift : { @_ };
  # ...
}
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update: possibly bad example - that's how I retrofit some of the legacy code at work. New code would inherit new() from Moose or Mouse :)