#   The variable $entity contains a MIME::Entity object returned from MIME::Parser 
#   and from this object, a MIME::Body object is retrieved for content manipulation.

my $body = $entity->body;
my $content = $body->as_string;

#   The content of the MIME component can now be modified as you see fit

$content =~ s/foo/bar/g;

#   The updated content must now be written back into the MIME message component - 
#   This is performed through manipulation of the MIME::Body object.

my $io = $body->open('w');
$io->print( $content )
$io->close;

#   At this point, you may need to re-synchronise the headers of the MIME::Entity 
#   object - This method will add or update a Content-Length header within the 
#   MIME component.

$entity->sync_headers(
        'Length'        =>  'COMPUTE',
        'Nonstandard'  =>  'ERASE'
);