Equivalent of mutt's nomark_old

In mutt, when you open a folder, and leave without reading a given New
message, by default it marks it as "O" rather than "N" the next time
you come back.

Opening the folder with Mail::Box makes a similar thing happen.

However, in mutt there's a nomark_old configuration option that stops
this happening, and keeps any message you haven't yet read as "N(ew)".

Is there an equivalent with Mail::Box?

Tony