You might want to replace your $sth->execute() with the following:
    unless ($sth->execute()) { die $dbh->errstr() }
This will give you an error if your sql statement is bad for some reason.

If you're not already doing it try:
    use CGI::Carp qw(fatalsToBrowser);
This will send all your errors to the browser. Remember to remove this line before the script goes live.

mr greywolf