sub import {
    my ( $self, $flag ) = @_;
    
    if ( defined $flag and $flag eq ':modern' ) {
        if ( defined $MODE ) {
            if ( $MODE == 0 ) {
                warn "Note: Foo is in classic mode\n"
            }
        }
        else {
            $MODE = 1;
            Foo-> export_to_level( 1, 'Foo', keys %const );
            
            no warnings 'redefine'; 
            *Foo::m2 = \&Foo::Modern::m2;
            # etc.
            # longer list
        }
    }
    else {
        if ( defined $MODE ) {
            if ( $MODE == 1 ) {
                warn "Note: Foo is in modern mode\n"
            }
        }
        else {
            $MODE = 0
        }
    }
}