FWIW, like suspected, ${^WARNING_BITS} is a pure compile time effect, no dynamic scope possible.

use strict;
use warnings;
use Data::Dump qw/pp dd/;

my ($before,$after);

BEGIN { $before = ${^WARNING_BITS} }


no warnings 'exiting';

BEGIN { $after = ${^WARNING_BITS} }

my $diff = $before ^ $after;

warn unpack ('B*',$diff);


{
    use warnings;
    BEGIN { ${^WARNING_BITS} = $after };  # this only works at compile
+ time!!!
    sub test {last };
    test();
}
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