$folder->message(3)->destruct;
See SYNOPSIS in Mail::Box::Message
When a message folder is read, each message will be parsed into Perl structures. Especially the header structure can consume a huge amount of memory (message bodies can be moved to external temporary files). Destructed messages have forcefully cleaned-up all header and body information, and are therefore much smaller. Some useful information is still in the object.
BE WARNED: once a message is destructed, it cannot be revived. Destructing enforces irreversable deletion from the folder. If you have a folder opened for read-only, the message will stay in that folder, but otherwise it may be deleted.
See DESCRIPTION in Mail::Box::Message