Various dbm algorithms will free space by marking sections as free and then reuse that space on need. So just because you are adding data doesn't mean that it will need to grow.

However you should know that SDBM_File is the worst of the dbm options available in Perl. It has a number of nasty limits, most importantly key/value pairs cannot exceed a fairly small size. If you can I strongly recommend using DB_File or GDBM_File. (I also recommend keeping a backup in plain text, but that is another story.)