Does anybody here know the difference between MARC21 and USMARC?

I am munging sets of MARC bibliographic data from a III catalog with
holdings data from the same. I am using MARC::Batch to read my bib' data
(with both strict and warnings turned off), insert 853 and 863 fields, and
writing the data using the as_usmarc method. Therefore, I think I am
creating USMARC files. I can then use marcdump to... dump the records. It
returns 0 errors.

We are then attempting to feed these files to Ex Libris's Primo but it
chokes on my data with the following error complete with spelling mistakes:

  2009-01-21 16:47:33,369 ERROR [t-SplitRecords] [c-RecordSplitter]
  - An exception occured while splitting record. error message is
  Content is not allowed in prolog.

Apparently we need to tell Primo that my data is one of the following
formats:

  MARC21
  MAB
  DANMARC2
  UNIMARC
  MARC Exchange

In what way is USMARC different from any of the formats above, and is it
possible to use the MARC::Record family of modules to write flavors of MARC
other than USMARC.

(Alternatively, maybe instead I need to turn strict and warning on and
filter out any bad records. Hmmm... )

-- 
Eric Lease Morgan
University of Notre Dame