my $oui_lookup;

sub oui_lookup {
    my $newval = shift;

    # validation, if necessary

    if ( defined $newval ) {
        $oui_lookup = $newval;
    }
    $oui_lookup;
}

##</code><code>##

my $old_lookup = oui_lookup();
my $new_lookup = oui_lookup( $some_new_val );