The warnings pragma is lexical, and somehow it seems inelegant to wrap an entire file's code in an unless statement. Probably a cleaner approach would be the if pragma:
use if ! $MODE eq 'PRODUCTION', warnings;
Personally I think I would prefer figuring out why there are warnings being generated in the first place, fix those that need fixing, and for the rest...
no warnings 'warning type';
# Code that generated the useless warning here
...but I do understand that there are times when enough thought has been put into it, nobody who understands the warning would see them anyway, and ultimately it's just time to silence them all and move on.