three-level.pl

#!/usr/bin/env perl
use v5.10;
use strict;
use warnings;

package Bar;

use warnings::register;

sub hello {
    warnings::warnif( "problem" );
    print "Hello\n";
}

package Foo;

sub foo_hi { Bar::hello() }

package Baz;

use warnings FATAL => 'all';

sub baz_hi { Foo::foo_hi }

package main;

Baz::baz_hi();