Are you sure that you're checking error message of the right call?

The DBI docs say this :-

  
$str = $h->errstr;

    Returns the native database engine error message from the last DBI
+ method called.
    ...
and
$DBI::err
    Equivalent to "$h->err".

   $DBI::errstr
    Equivalent to "$h->errstr".

   $DBI::state
    Equivalent to "$h->state".
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So didn't you just check if the last_insert_id call succeeded?

You need to check the err and maybe the state after the execute. So at least something like

$rv = $sth->execute   or die $sth->errstr;
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BTW it's probably worth asking this question on the posrgresql mailing list or IRC, as they'll know the best way to do this and it's more of a DB problem than a perl one.