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 my $dbh = DBI->connect( 'DBI:mysql:mysql', 'username', 'password' )
           || die $DBI::errstr;
 $dbh->do( 'use ' . DBNAME );

 use constant DSN => 'DBI:mysql:database=devel';
 ...
 my $dbh = DBI->connect( DSN, 'username', 'password' )
           || die $DBI::errstr;