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  package Exported;

...
      my ( $key ) = @_;
      return $dbh->selectrow_hashref( ... )
  }

  my $dbh = DBI->connect( ... );
  set_dbh( $dbh );
  my $answer = do_query( $key );

  package MyQuery;

...
      my ( $self, $key ) = @_;
      return $self->{dbh}->selectrow_hashref( ... );
  }

  my $dbh = DBI->connect( ... );
  my $query_obj = MyQuery->new( $dbh );
  my $answer = $query_obj->do_query( $key );

  package Exported;

...
      my ( $key ) = @_;
      return $dbh->selectrow_hashref( ... );
  }