Do you have the RaiseError attribute set to 1? If not, it could be possible that you previously assigned $sth to a *different* statement handle; then you tried to prepare this one, and there was an error, but you didn't see it, because you're not checking for errors. Could it be possible that $sth is still referencing the old statement handle, then? And that that old statement handle has 5 bind variables?

It sounds kind of farfetched, I know. :)

Anyway... try checking for errors on your prepare:

    $sth = $dbh->prepare($SQL_STATEMENT)
        or die "Can't prepare $SQL_STATEMENT: ", $dbh->errstr;
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Or turn on RaiseError, if you don't already have it on.

I've used this construct many, many times, so I'd say it's definitely something weird in your situation.