$ cat Foo.pm
package Foo;
use warnings::register;
sub bar { warnings::warnif("Test"); }
1;

$ perl -e'use Foo; Foo::bar();'

$ perl -e'use warnings; use Foo; Foo::bar();'
Test at -e line 1

$ perl -e'use Foo; use warnings; Foo::bar();'
Test at -e line 1