use My::Config;
no strict 'refs';
*'My::Config::DB_NAME' = sub { 'new_db' };

##</code><code>##

$ perl -MO=Deparse -e '
> package Foo;
> sub bar () { "baz" }
> package main;
> print Foo::bar(), "\n";
> print Foo->bar(), "\n";
> '
-e syntax OK
print 'baz', "\n";
print 'Foo'->bar, "\n";