I dug around in the source of Email::MIME and Email::Simple a bit. Email::MIME inherits from Email::Simple. Email::Simple has a body_set function that sets an object var, "body" (key in blessed hash ref). The "as_string" method in Email::MIME overrides the one in Email::Simple. The one in Email::MIME looks at the "body_raw" object var (key in blessed hash ref). So when you call body_set(), it sets "body" but when you call as_string, it doesn't look for "body" it looks for "body_raw" instead.

There are a number of things you can do. You can subclass Email::MIME and create a sub, body_set, that sets "body_raw" AND "body" so you're compatible with both. Or you can override as_string to print the one from body_raw instead. Instead of using "as_string" you can just print the headers then body yourself, using the headers and "body" joined by a newline.

From Email::MIME:

sub as_string {
  my $self = shift;
  return $self->__head->as_string
    . ($self->{mycrlf} || "\n")  # XXX: replace with ->crlf
    . $self->body_raw;
}
[download]

From Email::Simple:

sub as_string {
  my $self = shift;
  return $self->header_obj->as_string . $self->crlf . $self->body;
}
[download]