Your class doesn't need to inherit from DBI, it just needs to return a database handle. I've written lots of wrappers that connect to some default database in some default way that goes something like:

package MyDBI;

use DBI;

sub connect {
  ...get args
  my $dbh;
  ...do stuff
  $dbh = DBI->connect(...);
  ...do more stuff
  return $dbh;
}
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