sub addfor{
   my ($mainwidget,$entryVariable) = @_;
   unless (grep { $_ eq $entryVariable } @history) {
      $widget{'forwardlistbox'}->insert(end => $entryVariable);
      push @history, $entryVariable;
   }
}

##</code><code>##

use List::Util 'first';
sub addfor{
   my ($mainwidget,$entryVariable) = @_;
   unless (first { $_ eq $entryVariable } @history) {
      $widget{'forwardlistbox'}->insert(end => $entryVariable);
      push @history, $entryVariable;
   }
}