Setting/Changing header value

Didn't see it in the docs and rather than take a chance of breaking my 
running script I will ask here first.

Can the set method be used to change the Status of a (mbox format) 
message?

Here is how I open the folder and get the message header.

my $mgr = Mail::Box::Manager->new;

my $folder = $mgr->open(/path/to/mbox/formated/folder);

foreach my $msg ($folder->messages) {

    my $head = $msg->head;

### I'd like to do this
    $msg->set('Status') = 'R';
### and based on my thoughts later in this message
    $msg->set('X-Report-Processed') = 'T';
}

And if this code is even close to being correct what are the valid values
for the Status?  Could I 'create' my own status and not break too many
mail clients if the message escapes back out into the wild?
   Probably an even better idea is to create a custom header that I could
query to see if the message has already been processed.

Pointers, suggestions, clue-sticks? 


TIA,
Rod
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