sub unpack_named_on_the_fly {
  my ($pos1 ,$pos2 ,%arg) = @_;        
 
  $pos1     //  die("Missing arg!");   
  $pos2     //= 42;                  
  my $name1   = $arg{name1} // die("Missing arg!"); 
  my $name2   = $arg{name2};
 
  # ...etc
}