I had a similar problem. Looks like that when a script runs fetchrow_array and there is no more lines to be returned, DBD kill the cursor and , so, any try to access methods or attributes of the statement handle ($sth) will raise the error "fetch() without execute()".

I solved my problem with a conditional :

$sth->execute() or die "Error executing [$sqlcmd] : " . $dbh->errstr;
 $end_fetch = 0;
 do {
   if (! $end_fetch) {
      ($key_col, $col1, $col2, $col3) = $sth->fetchrow_array;
      if (! defined $key_col) {
         $end_fetch = 1;
      }
   }
...
} while ($some_condition);
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