Would calling the save_config from a destructor on the object work for you? Something like the following added to package FooBar:

#untested
sub DESTROY {
    my $self = shift;
    $self->save_config;
}
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-rattus
__________
He seemed like such a nice guy to his neighbors / Kept to himself and never bothered them with favors
- Jefferson Airplane, "Assassin"