update mytable set ? = 'blah'; # disallowed
update mytable set myfield = ?; # allowed
####
my %user_info;
my $query = "update tUser Set ".
(join ", " map {"$_ = $user_info{$_}"} keys %user_info).
" where UserID = $user_info{UserID}";
##
##
# Warning, untested code, to be used for inspiration only
my %user_info;
my $query = "update tUser Set ".
(join ", " map {"$_ = ?"} sort keys %user_info).
" where UserID = $user_info{UserID}";
my $sth = $dbh->prepare($query) or die;
$sth->execute(map {$user_info{$_}} sort keys %user_info);