I like use Carp; combined with a sensible combination of the Carp methods, using non-fatal warnings for some things, and using fatal warnings for most things (especially during development since you WANT the program to die so you have to fix the error).

Using Carp allows you to override the error handlers later without too much difficulty and substitute your own user-friendly error messages for production.

Also, Carp does some really nice things with stack tracing when you are using OO Perl.