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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

...
    " where UserID = $user_info{UserID}";
my $sth = $dbh->prepare($query) or die;
$sth->execute(map {$user_info{$_}} sort keys %user_info);