my %defaults = (
        data_source_name => \&database_data_source_name,
        username => \&database_username,
        password => \&database_password,
        taint => \&database_taint,
);

# and later:

my $dbh = DBI->connect (
               map { 
                    exists $arguments{$_} ?
                    $arguments{$_}  : 
                    $defaults{$_}() 
               } 
               qw(data_source_name username password taint)
          ) or die "Could not connect to the database: ".
                    "$DBI::errstr";

# and somewhere else:
sub database_data_source_name {
       # read stuff from file..
       # do other stuff
       return #something
}