The OP had this: 
$cols[0] = undef ;  # gives default value of that column

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which I have shown to be untrue. Doing that WILL set it to NULL.
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In reply to Re^5: MySQL: placeholders and NULL values by japhy
in thread MySQL: placeholders and NULL values by jeanluca

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