perl return codes - Mailing list pgsql-interfaces

I am writing a few perl scripts accessing postgres.   I have used the
examples in Bruce Momjian's book, where command results are tested.
(Variables such as PGRES_CONNECTION_OK and PGRES_TUPLES_OK are used).
Where can I find documentation on these types of variables, for other
commands?  (source is okay, I just cannot find it).  

thanks,
jr
john riehl