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();
}