I have tried a number of approaches to this error checking problem. None of them completely satisfied me. But for the moment here is the one that seems to work best.

What I do is having a function that does destructive manipulation of my arguments: 

# Takes a hashref, a key name, and an optional default.
# Removes that key from the hash, and returns the value or
# the default.  Blows up if there is no value or default.
sub excise_arg {
  my $args = shift;
  my $arg_name = shift;
  if (exists $args->{$arg_name}) {
    return delete $args->{$arg_name};
  }
  elsif (@_) {
    return shift;
  }
  else {
    confess("Missing required argument '$arg_name'");
  }
}

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Call it without a default argument and you have a required key. Put in the default and it is optional. And since it is destructive, it gets rid of the keys and I can use this for a typo check. 
# Takes a hashref.  Verifies that it is empty
sub assert_args_done {
  my @left = keys %{ $_[0] };
  if (@left) {
    confess("Unexpected arguments '@left' left over");
  }
}

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And now in a function I can do this: 
sub some_func {
  my $args = { @_ };
  my $name = excise_arg($args, 'name');       # required
  my $age = excise_arg($args, 'age', undef);  # optional
  assert_args_done($args);
  # Rest of the code here.
}

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And if I want to take an existing function and wrap it in one that can handle some things itself and wraps the rest, it is easy. I just excise a few arguments and then pass the rest through untested. As long as they are tested somewhere, typos get checked.

If anyone has alternate suggestions for how to handle this problem, I am open. This seems to work pretty well, but I have tried several things, and I don't claim that this is perfect.

UPDATE
Thanks Hofmator for catching my obvious typo. That is what I get for typing something up off of the top of my head. That is also why I use strict. :-)

In reply to Re (tilly) 2: passing subroutine arguments directly into a hash by tilly
in thread passing subroutine arguments directly into a hash by c

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