I believe MySQL doesn't like to allow you to bind to column names.

update mytable set ? = 'blah'; # disallowed
update mytable set myfield = ?; # allowed
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Also, looping over multiple fields and executing one query for each is most likley going to be a huge performance loss. You'd probably be better off constructing one query, even if you need to prepare it every time. You might try something like this:

my %user_info;

my $query = "update tUser Set ".
    (join ", " map {"$_ = $user_info{$_}"} keys %user_info).
    " where UserID = $user_info{UserID}";
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This uses interpolation instead of query binding. If you always specify your mysql columns in the same order, you could use binding instead:

# 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);
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Alan