You are viewing the version of this documentation from Perl 5.31.11. This is a development release of Perl.
${^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.

This variable was added in Perl v5.6.0.