I'd say that this is precisely the situation that Carp is designed for--use carp or croak rather than warn and die, and they'll know which invocation of the function they messed up, which is generally fairly useful information.

Alternatively, set $@, return undef, and let the caller figure out the problem. :-)

I rarely return undef explicitly, as it tends to do the wrong thing in list context. return by itself is shorter and smarter.