From DBD::Sybase

The last_insert_id() call is simply a wrapper around a "select @@ident
+ity" query. To be successful (i.e. to return the correct value) this 
+must be executed on the same connection as the INSERT that generated 
+the new IDENTITY value. Therefore the statement handle that was used 
+to perform the insert must have been closed/freed before last_insert_
+id() can be called. Otherwise last_insert_id() will be forced to open
+ a different connection to perform the query, and will return an inva
+lid value (usually in this case it will return 0).
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Try adding ->finish()

for (my $x=0;$x < 5;$x++){ 
  $insertStatement->execute() or die($DBI::errstr); 
  $insertStatement->finish();
  my $id = $dbh->last_insert_id(undef,undef,undef,undef) 
           or die "no insert id $DBI::errstr $!"; 
  print "ID :",$id; 
  update1($id); 
}
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poj