I'd agree with Fletch to some extent but I would recommend Email::Folder to read the mailbox and Email::LocalDelivery to 'deliver' it to the new mailbox:

use strict;
use warnings;
                                                                      
+          
use Email::Folder;
use Email::LocalDelivery;
                                                                      
+          
my $count = 0;
my $box_size = 100;
                                                                      
+          
my $boxbase = '/home/jonathan/tmp/testmail/spam';
                                                                      
+          
my $box = Email::Folder->new('/home/jonathan/mail/spam');
while ( my $mail = $box->next_message )
{
   my $newbox = $boxbase . int($count++ / $box_size);
    Email::LocalDelivery->deliver($mail->as_string(),($newbox));
}
[download]

/J\