$sql = "SELECT field_1, field_2 FROM myTable";
$sth = $dbh->prepare($sql) or die("Could not prepare!" . $dbh->errstr);
$sth->execute() or die("Could not execute!" . $dbh->errstr);
while (($val_1, $val_2)  = $sth->fetchrow_array()) {
  $myhash{$val_1} = $val_2;
}
$sth->finish;
foreach $val_1 (keys %myhash) {
   print "myhash{$val_1} = $myhash{$val_1}\n";
}


##</code><code>##

if ($val_2) {
   $myhash{$val_1} = $val_2;
}
else {
   $myhash{$val_1} = '';
}