my %col;
$col{foo}=1;
$col{bar}=undef;


my %vbph;
my @vbval;
foreach(sort keys %arg){
  if(defined $arg{$_}){
     $vbph{$_}='?';
     push @vbval,$arg{$_};    
  }else{
     $vbph{$_}='NULL';
  }
}
my $sql='update baz set '.
          join ', ',(map{$_.'='.$vb{$_}} sort keys %arg));
my $sth=$dbh->prepare_cached($sql);
$sth->execute(@vbval);