my $f = Mail::Message::Field::Unstructured->new(Comments => 'hi!');
See SYNOPSIS in Mail::Message::Field::Full
Unstructured fields do contain information which is not restricted in any way. RFC2822 defines some unstructured fields, but by default all unknown fields are unstructured as well. Things like attributes and comments have no meaning for unstructured fields, but encoding does.
See DESCRIPTION in Mail::Message::Field::Full