Investigating ${^WARNING_BITS} in Perl 5.26.2

main.pl

use 5.014;
use warnings; 
use lib '.';
use W; 

say "running"; 

no warnings; 
W->report;        # 0000...

use warnings; 
W->report;        # 1010...

W.pm

package W; 
#use Data::Dumper;
use strict;
use 5.014;

sub import { 
    say "Compile time: ", unpack('b*', ${^WARNING_BITS}); 
        # ${^WARNING_BITS} is only available at compile time per 
        # https://www.perlmonks.org/?node_id=611472
}

sub report {
    #my(@callers_bitmask) = (caller(0))[9];     # What warnings.pm does

    my @caller0 = caller(0);    # Who called us
        # Must be caller(0), not just caller(), because without a parameter
        # caller only returns the package, filename, and line number.
    my $callers_bitmask = $caller0[9];

    #say Dumper(\$callers_bitmask);
    say "Runtime at $caller0[1]:$caller0[2]: ", unpack('b*', $callers_bitmask);
        # E.g., "Runtime at main.pl:9: ..."
}

1;

${^WARNING_BITS} is only available at compile time per https://www.perlmonks.org/?node_id=611472 . This matches my experience. As far as I can tell, the Perl source makes ${^WARNING_BITS} magical. Then, when the warnings pragma changes ${^WARNING_BITS} at compile time, PL_compiling.cop_warnings is updated. Somehow, PL_compiling.cop_warnings must be getting stashed away where caller() can access it at runtime.

Example output

perl main.pl produces:

Compile time: 1010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010
running
Runtime at main.pl:9: 0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

(this is after no warnings)

Runtime at main.pl:12: 1010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010101010

(this is after use warnings)