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.