use Scope::Guard qw(guard);
{ 
    my @guards;
    for (keys %handler) {
        # Save the old state:
        my($k,$p) = ($_,\&$_);
        push @guards, guard { 
            # restore the old state
            *$k = $p;
        };
        # Install our new handler
        *$_ = \&$handler{$_};
    }
    
    # "execute" the config
    # ...
}
# Scope::Guard executes our guard blocks here