Looks very useful, gOn
++. Am I right in thinking that this only applies to DBI? If so, I'd really suggest a name in the DBIx::* hierarchy since Data:: applies, in my mind to all kinds of data, not just DBI. I suggest you drop a note to firstname.lastname@example.org mentioning the url of this node so the folks over there can get in on the RFC. Also, you should make it explicit what happens in these cases: a) when you try to merge a column of one type in a db with a column of another type in the second db and b) when the length of the data in one db is different from that in the second e.g. you try to put a VARCHAR(50) value into a VARCHAR(10) column.
Also, is this meant to only work on two database in the same RDBMS or is it meant to be able to go, e.g. from a MySQL database to a Pg database?