I think you are bypassing the documented interface of Mail::Header by accessing its internal data like that (mail_hdr_hash). The following gives you the headers with the keyword removed:

push @hlines, $header->get('To');
push @hlines, $header->get('Cc');
push @hlines, $header->get('Bcc');
[download]

According to the Mail::Header documentation, get() in array context returns "a list of all the text from all the instances of TAG". Which should be what you are looking for.

--ZZamboni