One point, if a message is HTML and has an attachment then the $ent->effective_type would be multipart/mixed and then $ent->parts(0)->effective_type would be multipart/alternative and then the sub-parts ($ent->parts(0)->parts(x)->effective_type) of this would be text/plain(0) and text/html(1).

So you would need to add this after testing for multipart/alternative :)

if ($ent->effective_type eq "multipart/mixed"
    and $ent->parts(0)->effective_type eq "multipart/alternative"
    and $ent->parts(0)->parts == 2
    and $ent->parts(0)->parts(0)->effective_type eq "text/plain"
    and $ent->parts(0)->parts(1)->effective_type eq "text/html") {

    my $newent = MIME::Entity->build(Data =>
                 $ent->parts(0)->parts(0)->body_as_string);
    $ent->parts(0)->parts([$newent]);
    $ent->parts(0)->make_singlepart;
    $ent->parts(0)->sync_headers(Length => 'COMPUTE', Nonstandard => '
+ERASE');
    $ent->sync_headers(Length => 'COMPUTE', Nonstandard => 'ERASE');
}
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-jeffphil