given 
use warnings; # implies 'all'
use Some::Module;  # does a warnings::register
Some::Module::badcall();

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The call to badcall() will see that that module's warnings are enabled. Looking at the code in use warnings and use warnings::register, I see that adding a category will extend the mask for 'all' to include the new bit, but it can't do a thing about the existing values of ${^WARNING_BITS}. Shouldn't that new bit still be off in this scope?

In reply to How does this code work (warnings.pm)? by John M. Dlugosz

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