I ran some tests, and
warnif (and similar constructs) is the only thing affected.
Update: Specifically,
- no warnings 'OtherModule'; has no effect on builtin warnings.
- use warnings 'OtherModule'; has no effect on builtin warnings.
- use warnings 'OtherModule'; has no effect on warn warnings.
- no warnings 'OtherModule'; has no effect on warn warnings.
- use warnings 'OtherModule'; in the caller enables warnings::warnif warnings in the module.
- no warnings 'OtherModule'; in the caller disables warnings::warnif warnings in the module.
- use warnings 'OtherModule'; in the module has no effect.
- no warnings 'OtherModule'; in the module has no effect.
- use warnings; in the caller enables warnings::warnif warnings in the module.
- no warnings; in the caller disables warnings::warnif warnings in the module.
- use warnings; in the module has no effect on warnings::warnif warnings in the module.
- no warnings; in the module has no effect on warnings::warnif warnings in the module.