Rather than setting %args from @_ and then adding in defaults, I tend to use something along the lines of:

sub foo {
  my @defaults = ( Index => [],
                                 weeble => "floop", );
  my %args = ( @defaults, @_ );

  for( @{ $args{ Index } } ) {
    ## ...
  }
}
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You might also want to check out Params::Validate, which provides checking of parameters and allows you to specify default values where required.