#${^WARNING_BITS}

The current set of warning checks enabled by the use warnings pragma. It has the same scoping as the $^H and %^H variables. The exact values are considered internal to the warnings pragma and may change between versions of Perl.

Each time a statement completes being compiled, the current value of ${^WARNING_BITS} is stored with that statement, and can later be retrieved via (caller($level))[9].

This variable was added in Perl v5.6.0.

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