if( $form->submitted ) {
        if( $form->submitted eq 'Cancel' ) {
            return $self->mainmenu_form_generate( undef );
        }
        if( $form->submitted eq '< Back' ) {
            # User wants to return to previous form
            return $self->upload2_form_generate( undef );
        }
... other code to process 'Continue' button ...
    }

##</code><code>##

my  $JSNoValdtnOnCancelBack = <<EOJSFUNC;
    if (form._submit.value == 'Cancel') {
       // skip validation since we're cancelling
       return true;
    }
EOJSFUNC

... later ...
  $o .= $form->render(
          template   => {
            type      => 'TT2',
                :::::
          },
          jsfunc    => $JSNoValdtnOnCancelBack,
      );