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

##</code><code>##

sub database_username {
    return "fred";
}

##</code><code>##

my %defaults = (
      data_source_name => ".....",
      username => "fred",
      password => "fred",
      taint => { Taint => 1 }
);

# then later...
%arguments = (%defaults, %arguments);

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

##</code><code>##

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
}

##</code><code>##

           map {
                   exists $arguments{$_} ?
                   $arguments{$_}  :
                   ${\"database_$_"}->()
           }