Whether fetchrow_array gets all the selections at once or not depends on the DBD module. IIRC, mysql grabs them all, but an rdbms with cursors could deliver one at a time. That comes at a cost to speed, but so does running out of memory.

Set RaiseError => 1 in the db handle to get handier error handling.

Update: If yours is MySQL, you can SELECT foo, bar, baz INTO OUTFILE '/path/to/flatfile.txt' . . . , provided you have FILE privilege.