AFAIK you should not be forced to use prototypes anywhere where you didn't in 5.004.

Two things that will help you find the cause of the problem:

1. Search on the exact error message in google, for instance, and/or use the diagnostics module. Searching on google will have the advantage that it will probably also explain error messages that aren't caused by perl, since perl doesn't generate any warning/error like the one you've posted here.

2. If that doesn't work, post the exact error message here and include (some of) the code that demonstrates the problem.