Learn them on an "as needed" basis and be willing to look in perlfunc (and on CPAN) when you feel the need.

That said, use tie with a module for a specific dbm rather than dbmopen. Unless you can explain exactly how dbmopen can result in an innocent appearing upgrade causing your databases to all become corrupted, and you are willing to explain why you know it won't happen to you and your data, I would avoid it. (Hint: dbmopen is just a wrapper around tie and AnyDBM_File.)