I like the subroutine idea for specifying hints. @_ could be the return values, and $_ aliased to $_[0]. Perhaps something like

autodie::hints->set_hints_for(
  \&foo,
  scalar => sub { ! $_ },
  list   => sub { ! @_ or @_ == 1 and ! defined },
);

autodie::hints->set_hints_for(
  \&copy,
  any => sub { ! $_ },
);

autodie::hints->set_hints_for(
  'open',
  any => sub { ! defined },
);
[download]

would work.

This makes it easy to define common behaviours and to compose them in an arbitrary way.

use autodie::hints qw/ DEFAULT_SCALAR DEFAULT_LIST /;
autodie::hints->set_hints_for(
  \&foo,
  scalar => DEFAULT_SCALAR,
  list   => DEFAULT_LIST,
);

use autodie::hints qw/ COND1 COND2 COND3 /;
autodie::hints->set_hints_for(
  \&foo,
  any => sub { &COND1 and &COND2 or &COND3 },
);
[download]

If access to e.g. the arguments to the function is needed (for e.g. unlink) then that would have to be provided some other way, for instance via a localized global variable in autodie::hints.

Just an idea ...

lodin