Here's one way to do it, if you just want a conditional:

sub make_report1 {

  my $dbh = shift;
  my $newdbh=0;

  if (!defined $dbh) {
    $dbh = &_get_dbh;
    $newdbh=1;
  }

  ...

  # close the database connection if opened here
  $dbh->disconnect() if $newdbh;

}
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