I keep my connection info in a config file, so I'll have something like library.site.conf that includes

DB_NAME         = library
DB_HOST         = localhost
DB_USER         = username
DB_PASS         = password
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Then, in my base DBI package, I have:

package MyDatabase::DBI;
use base qw(Class::DBI::mysql);
my %config;
use Config::General;
my $conf = new Config::General(
  -ConfigFile => '/usr/local/configfiles/library.site.conf',
  -InterPolateVars => 1
                              );
%config = $conf->getall;

my $dsn = "DBI:mysql:host=$config{DB_HOST};database=$config{DB_NAME}";
my $db_user = $config{DB_USER};
my $db_pass = $config{DB_PASS};

__PACKAGE__->set_db('Main', $dsn, $db_user, $db_pass);
# etc.
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There are a ton of similar ways to do this.