Why does dbmopen create two files?

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

When I create a database using dbmopen, e.g.

dbmopen( %DB, $datapathname, 0666 ) or die;
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dbm creates two files rather one:

data.dir
data.pag
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Why is that?

Originally posted as a Categorized Question.