See SYNOPSIS in Mail::Message::Head
This class implements a pure IMAP4 protocol interface, where as few data is retreived from the header as possible. This may look nice to you, but is not sufficient for many tasks. For instance, you cannot removed or modify fields this way.
Change Mail::Box::IMAP4::new(cache_head) to YES
or DELAY
, to
get a message header which is capable of performing all possible
games with headers. On the other hand: the other settings are not
100% safe...
See DESCRIPTION in Mail::Message::Head