use strict;
use warnings;
use DBI;
use File::Copy qw(copy);

my $dsn = 'DBI:ODBC:somedb';
my $db_user = '';
my $db_pass = '';

my $dbh = DBI->connect($dsn,$db_user,$db_pass ) or die $DBI::errstr;

if (1) {
  # This causes the warning
  my $loh = $dbh->selectall_hashref('select 1 as a, 2 as b','a');
}
if (0) {
  # This causes the warning
  my $sth = $dbh->prepare('select 1 as a, 2 as b');
  $sth->execute();
  my $loh = $sth->fetchall_hashref('a');
  $sth->finish;
}
if (0) {
  # This does not cause the warning
  my $aoa = $dbh->selectall_arrayref('select 1 as a, 2 as b');
}
if (0) {
  # This does not cause the warning
  my $sth = $dbh->prepare('select 1 as a, 2 as b');
  $sth->execute();
  my $hash = $sth->fetchrow_hashref();
  $sth->finish;
}
copy('a','b') or die "Failed to copy a to b. $!\n";
$dbh->disconnect;